[libcxx] Switch to release branch runners

This ensures that if/when we bump the toolchain versions in the main
container that we do not break tests on the release branch.

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

@@ -36,7 +36,7 @@ concurrency:
 jobs:
   stage1:
     if: github.repository_owner == 'llvm'
-    runs-on: llvm-premerge-libcxx-runners
+    runs-on: llvm-premerge-libcxx-release-runners
     continue-on-error: false
     strategy:
       fail-fast: false
@@ -73,7 +73,7 @@ jobs:
             **/crash_diagnostics/*
   stage2:
     if: github.repository_owner == 'llvm'
-    runs-on: llvm-premerge-libcxx-runners
+    runs-on: llvm-premerge-libcxx-release-runners
     needs: [ stage1 ]
     continue-on-error: false
     strategy:
@@ -147,19 +147,19 @@ jobs:
           'generic-static',
           'bootstrapping-build'
         ]
-        machine: [ 'llvm-premerge-libcxx-runners' ]
+        machine: [ 'llvm-premerge-libcxx-release-runners' ]
         include:
         - config: 'generic-cxx26'
-          machine: llvm-premerge-libcxx-runners
+          machine: llvm-premerge-libcxx-release-runners
         - config: 'generic-asan'
-          machine: llvm-premerge-libcxx-runners
+          machine: llvm-premerge-libcxx-release-runners
         - config: 'generic-tsan'
-          machine: llvm-premerge-libcxx-runners
+          machine: llvm-premerge-libcxx-release-runners
         - config: 'generic-ubsan'
-          machine: llvm-premerge-libcxx-runners
+          machine: llvm-premerge-libcxx-release-runners
         # Use a larger machine for MSAN to avoid timeout and memory allocation issues.
         - config: 'generic-msan'
-          machine: llvm-premerge-libcxx-runners
+          machine: llvm-premerge-libcxx-release-runners
     runs-on: ${{ matrix.machine }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2