Switch testing to pre-installed versions of clang-14 (#2402)

We keep seeing fragility installing software, with both brew (e.g., the recent python issues) and apt.llvm.org (currently flaky).

At this point, images for both ubuntu and macos have versions of llvm-14 that seem to successfully compile:
https://github.com/carbon-language/carbon-lang/actions/runs/3474670746/jobs/5808098087

Although we may want to figure out a way to resume running llvm-15 so that we can see compatibility issues, this seems preferable for baseline testing in order to reduce maintenance churn.

In addition to the above changes, this also configures cancellation more precisely, and stops installing bazel/bazelisk (it should already be preinstalled).

.github/workflows/tests.yaml

@@ -25,11 +25,20 @@ on:
       - '!CODEOWNERS'
       - '!.git*'
 
+# Cancel previous workflows on the PR when there are multiple fast commits.
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   test:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        # At present, these images are newer than "latest". We use them to test
+        # against more recent tooling versions.
+        # https://github.com/actions/runner-images
+        os: [ubuntu-22.04, macos-12]
         build_mode: [fastbuild, opt]
     runs-on: ${{ matrix.os }}
     steps:
@@ -50,49 +59,19 @@ jobs:
           # Match the min version listed in docs/project/contribution_tools.md
           python-version: '3.9'
 
-      # On macOS we need Go and to use it to install Bazelisk.
-      - uses: actions/setup-go@v3
-        if: matrix.os == 'macos-latest'
-      - name: Install bazelisk
-        if: matrix.os == 'macos-latest'
-        run: |
-          go get github.com/bazelbuild/bazelisk
-          echo "$(go env GOPATH)/bin" >> $GITHUB_PATH
-
-      # On macOS, use Homebrew to install a recent LLVM and Clang.
+      # Use LLVM 14 following:
+      # https://github.com/actions/runner-images/blob/main/images/macos/macos-12-Readme.md
       - name: Setup LLVM and Clang (macOS)
-        if: matrix.os == 'macos-latest'
-        env:
-          HOMEBREW_NO_INSTALL_CLEANUP: 1
-          HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK: 1
+        if: matrix.os == 'macos-12'
         run: |
-          echo '*** Updating brew'
-          brew update
-          echo '*** Installing Python'
-          # Explicitly use --overwrite because of file conflicts in
-          # /usr/local/bin with GitHub's installed version. If this stops being
-          # required, great!
-          brew install --force-bottle --overwrite python@3.11
-          echo '*** Installing LLVM deps'
-          brew install --force-bottle --only-dependencies llvm
-          echo '*** Installing LLVM itself'
-          brew install --force-bottle --force --verbose llvm
-          echo '*** brew info llvm'
-          brew info llvm
-          echo '*** brew config'
-          brew config
-          echo '*** Updating PATH'
-          echo "$(brew --prefix llvm)/bin" >> $GITHUB_PATH
+          echo "$(brew --prefix llvm@14)/bin" >> $GITHUB_PATH
 
-      # On Ubuntu, use apt.llvm.org to install a recent LLVM and Clang.
-      - name: Setup LLVM and Clang (Ubuntu)
-        if: matrix.os == 'ubuntu-latest'
+      # Use LLVM 14 following:
+      # https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2204-Readme.md
+      - name: Setup LLVM and Clang (Linux)
+        if: matrix.os == 'ubuntu-22.04'
         run: |
-          wget https://apt.llvm.org/llvm.sh
-          chmod +x llvm.sh
-          sudo ./llvm.sh 15 all
-          rm llvm.sh
-          echo "/usr/lib/llvm-15/bin" >> $GITHUB_PATH
+          echo "/usr/lib/llvm-14/bin" >> $GITHUB_PATH
 
       # Print the various tool paths and versions to help in debugging.
       - name: Print tool debugging info
@@ -120,6 +99,16 @@ jobs:
           echo "$GCP_BUILDS_SERVICE_ACCOUNT" \
             | base64 -d > $HOME/gcp-builds-service-account.json
 
+      # We need to replace the `.` with a `_` for the build cache.
+      - name: Setup LLVM and Clang (macOS)
+        if: matrix.os == 'macos-12'
+        run: |
+          echo "os_for_cache=macos-12" >> $GITHUB_ENV
+      - name: Setup LLVM and Clang (Linux)
+        if: matrix.os == 'ubuntu-22.04'
+        run: |
+          echo "os_for_cache=ubuntu-22_04" >> $GITHUB_ENV
+
       # Add our bazel configuration and print basic info to ease debugging.
       - name: Configure Bazel and print info
         env:
@@ -129,7 +118,7 @@ jobs:
         run: |
           cat >user.bazelrc <<EOF
           # Enable remote cache for our CI.
-          build --remote_cache=https://storage.googleapis.com/carbon-builds-github-v${CACHE_VERSION}-${{ matrix.os }}
+          build --remote_cache=https://storage.googleapis.com/carbon-builds-github-v${CACHE_VERSION}-${{ env.os_for_cache }}
           build --google_credentials=$HOME/gcp-builds-service-account.json
 
           # General build options.