Adjust some build troubleshooting notes (#4471)

Came up due to [libc++ install
issues](https://discord.com/channels/655572317891461132/655577725347561492/1302023663155023905)

We've discussed clang version verification, and adding that as long as
I'm in here. The more significant bit is the libc++ check, which if it's
not installed should fail like:

```
(tons of output)
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/llvm-16/lib/clang/16/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
/usr/local/google/home/jperkins/.cache/bazel/_bazel_jperkins/85deb7d9d96f7e0e80b42618a55969d7/external/_main~clang_toolchain_extension~bazel_cc_toolchain/_temp:6:2: error: "No libc++ install found!"
#error "No libc++ install found!"
 ^
1 error generated.
ERROR: Analysis of target '//toolchain:toolchain' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.265s, Critical Path: 0.08s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully
```

pre-commit runs bazel, and GitHub runners have an old clang by default
(caught by the new check), so I'm installing here for a consistent
version.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

.github/workflows/pre_commit.yaml

@@ -26,6 +26,7 @@ jobs:
           # prettier-ignore
           allowed-endpoints: >
             *.dl.sourceforge.net:443
+            api.github.com:443
             bcr.bazel.build:443
             downloads.sourceforge.net:443
             files.pythonhosted.org:443
@@ -40,6 +41,13 @@ jobs:
 
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
       - uses: actions/setup-python@82c7e631bb3cdc910f68e0081d67478d79c6982d # v5.1.0
+
+      # Ensure LLVM is set up consistently.
+      - uses: ./.github/actions/build-setup-common
+        with:
+          matrix_runner: ubuntu-latest
+          remote_cache_upload: '--remote_upload_local_results=false'
+
       - uses: pre-commit/action@2c7b3805fd2a0fd8c1884dcaebf91fc102a13ecd # v3.0.1
 
       # We want to automatically create github suggestions for pre-commit file