Switch to llvm apt for ubuntu (#2160)

We've been encountering issues installing llvm via brew on linux, e.g.:
https://github.com/carbon-language/carbon-lang/runs/8258419618?check_suite_focus=true

```
==> Reinstalling 2 dependents with broken linkage from source:
llvm, mpdecimal
```

(but then llvm doesn't get reinstalled)

This should resolve it:
https://github.com/carbon-language/carbon-lang/runs/8260573590?check_suite_focus=true

.github/workflows/tests.yaml

@@ -62,34 +62,44 @@ jobs:
       # Setup to the latest LLVM and Clang release.
       #
       # Ideally we would use the pre-installed versions in the image, but the
-      # debian packages for LLVM-12 are broken due to several bugs:
-      # https://bugs.llvm.org/show_bug.cgi?id=43604
-      # https://bugs.llvm.org/show_bug.cgi?id=46321
+      # debian packages for LLVM-15 currently don't work for fastbuild.
       #
       # For now, we rely on Homebrew to manage installing a correctly built
       # toolchain. We also take some care to be as resilient as possible to
       # issues fetching and installing the toolchain.
-      - name: Setup LLVM and Clang
+      - name: Install Clang/LLVM using brew
         env:
           HOMEBREW_NO_INSTALL_CLEANUP: 1
+          HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK: 1
         run: |
+          echo '*** Updating brew'
           brew update
+          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
 
       # Print the various tool paths and versions to help in debugging.
       - name: Print tool debugging info
         run: |
+          echo '*** PATH'
           echo $PATH
+          echo '*** bazelisk'
           which bazelisk
           bazelisk --version
+          echo '*** python'
           which python
           python --version
+          echo '*** clang'
           which clang
           clang --version
+          echo '*** clang++'
           which clang++
           clang++ --version