ci: pin GitHub Actions macOS runners to macos-15

Co-authored-by: Michi Hoffmann <michael.hoffmann@sentry.io>

Author: sentry-junior[bot]

.github/workflows/ci.yml

@@ -374,7 +374,7 @@ jobs:
             ubuntu-24.04,
             ubuntu-22.04,
             ubuntu-22.04-arm,
-            macos-latest, # macOS arm64
+            macos-15, # macOS arm64
             macos-15-intel, # macOS x64
             windows-latest,
           ]