[llvm] [libc++] Pin down the OS version of macOS runners (PR #225536)

[Louis Dionne via llvm-commits]

https://github.com/ldionne created https://github.com/llvm/llvm-project/pull/225536

Otherwise, we sometimes end up using runners that don't have Xcode 26.5 on them. Pinning this allows bumping the version explicitly when we are ready to.

>From 791e6af8eed4ce330b87c4c7b0f6ba5119c3377b Mon Sep 17 00:00:00 2001
From: Louis Dionne <ldionne.2 at gmail.com>
Date: Tue, 22 Sep 2026 14:50:18 -0700
Subject: [PATCH] [libc++] Pin down the OS version of macOS runners

Otherwise, we sometimes end up using runners that don't have Xcode 26.5
on them. Pinning this allows bumping the version explicitly when we are
ready to.
---
 .github/workflows/libcxx-pr-conformance-tests.yaml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/libcxx-pr-conformance-tests.yaml b/.github/workflows/libcxx-pr-conformance-tests.yaml
index 21b4a65bd702a..6e333befa9b3c 100644
--- a/.github/workflows/libcxx-pr-conformance-tests.yaml
+++ b/.github/workflows/libcxx-pr-conformance-tests.yaml
@@ -230,7 +230,7 @@ jobs:
           xcode-version: '26.5'
         - config: apple-system-hardened
           xcode-version: '26.5'
-    runs-on: ["self-hosted", "macOS", "apple-runners"]
+    runs-on: ["self-hosted", "macOS", "apple-runners", "26.5"] # macOS 26.5, which has Xcode 26.5 too
     steps:
       - uses: actions/checkout at df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
         with: