Add a scheduled build for libc++ (llvm#73848)

We had a scheduled build on buildkite, we probably want something
testing head here as well.

Since this change just adds more tests, I think we can skip pre-commit  review.

[skip ci]

Author: EricWF

.github/workflows/libcxx-build-and-test.yaml

@@ -22,6 +22,9 @@ on:
       - 'runtimes/**'
       - 'cmake/**'
       - '.github/workflows/libcxx-build-and-test.yaml'
+  schedule:
+    # Run nightly at 8 AM UTC (or roughly 3 AM eastern)
+    - cron: '0 3 * * *'
 
 permissions:
   contents: read # Default everything to read-only