[llvm] [libc++] Run macOS CI on self-hosted runners instead of Github ones (PR #210124)

[Mishal Shah via llvm-commits]

================
@@ -216,18 +212,15 @@ jobs:
         #       macOS versions as a way to avoid rotting that configuration, but it doesn't provide a lot of additional
         #       coverage.
         - config: apple-system
-          os: macos-26
         - config: apple-system-hardened
-          os: macos-26
-    runs-on: ${{ matrix.os }}
+    runs-on: ["self-hosted", "macOS", "ARM64", "26", "26.5"]
----------------
shahmishal wrote:

```suggestion
    runs-on: ["self-hosted", "macOS", "apple-runners"]
```

https://github.com/llvm/llvm-project/pull/210124