Pin Swift macOS checks to macOS 15 for Xcode 16

`macos-latest-xlarge` now resolves to macOS 26, which ships only Xcode 26
(Swift 6.2) and no longer includes Xcode 16. The Swift-analysing PR checks
select Xcode 16 because CodeQL CLI versions before 2.24.0 only support Swift
up to 6.1, so `xcode-select -s /Applications/Xcode_16.app` fails on macOS 26.

Pin these checks to macOS 15, where Xcode 16 is still available, so we keep
testing the full matrix of supported CodeQL versions. This also pre-empts the
`swift-custom-build` check failing once plain `macos`/`macos-latest` migrates.

See actions/runner-images#14167.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: henrymercer

.github/workflows/__multi-language-autodetect.yml

@@ -2,8 +2,11 @@ name: "Multi-language repository"
 description: "An end-to-end integration test of a multi-language repository using automatic language detection"
 operatingSystems:
   - ubuntu
+  # Pin to macOS 15 rather than `macos-latest`: the older CodeQL CLI versions in the
+  # matrix only support Swift up to 6.1 (Xcode 16), which is not available on macOS 26
+  # (ships Xcode 26 / Swift 6.2). See https://github.com/actions/runner-images/issues/14167.
   - os: macos
-    runner-image: macos-latest-xlarge
+    runner-image: macos-15-xlarge
 env:
   CODEQL_ACTION_RESOLVE_SUPPORTED_LANGUAGES_USING_CLI: true
 installGo: true

.github/workflows/__swift-custom-build.yml

@@ -5,7 +5,11 @@ versions:
   - default
   - nightly-latest
 operatingSystems:
-  - macos
+  # Pin to macOS 15 rather than `macos-latest`: the `linked`/`default` CodeQL CLI versions
+  # are analysed with Xcode 16, which is not available on macOS 26 (ships Xcode 26 / Swift
+  # 6.2). See https://github.com/actions/runner-images/issues/14167.
+  - os: macos
+    runner-image: macos-15
 installGo: true
 installDotNet: true
 env: