Add some ls commands to help diagnose tool issues (#2540)

We're seeing some test runs where `clang` isn't in the path on macos. I suspect it's a bad image, and can't reproduce it, but want to add these commands to help provide debug info for potential future problems.

e.g., the `ls` output: https://github.com/carbon-language/carbon-lang/actions/runs/3963366092/jobs/6791110228

e.g., the bad image: https://github.com/carbon-language/carbon-lang/actions/runs/3963270727/jobs/6790912681

.github/workflows/tests.yaml

@@ -65,6 +65,10 @@ jobs:
         if: matrix.os == 'macos-12'
         run: |
           echo "$(brew --prefix llvm@14)/bin" >> $GITHUB_PATH
+          echo '*** ls "$(brew --prefix llvm@14)"'
+          ls "$(brew --prefix llvm@14)"
+          echo '*** ls "$(brew --prefix llvm@14)/bin"'
+          ls "$(brew --prefix llvm@14)/bin"
 
       # Use LLVM 14 following:
       # https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2204-Readme.md
@@ -72,6 +76,8 @@ jobs:
         if: matrix.os == 'ubuntu-22.04'
         run: |
           echo "/usr/lib/llvm-14/bin" >> $GITHUB_PATH
+          echo '*** ls /usr/lib/llvm-14/bin'
+          ls /usr/lib/llvm-14/bin
 
       # Print the various tool paths and versions to help in debugging.
       - name: Print tool debugging info