CodeQL: Run on macOS by default if the target language is Swift

CodeQL Swift analysis is best supported on macOS.

In preparation for CodeQL supporting Swift analysis in beta,
adjust the CodeQL starter workflow template to run the `swift` matrix job on `macos-latest`, and all other matrix jobs on
`ubuntu-latest`. This does not affect the matrix itself.

code-scanning/codeql.yml

@@ -23,7 +23,7 @@ on:
 jobs:
   analyze:
     name: Analyze
-    runs-on: ubuntu-latest
+    runs-on: ${{ (matrix.language == 'swift' && 'macos-latest') || 'ubuntu-latest' }}
     permissions:
       actions: read
       contents: read
@@ -51,7 +51,7 @@ jobs:
         # By default, queries listed here will override any specified in a config file.
         # Prefix the list here with "+" to use these queries and those in the config file.
 
-        # Details on CodeQL's query packs refer to : https://docs.github.com/en/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-code-scanning#using-queries-in-ql-packs
+        # For more details on CodeQL's query packs, refer to: https://docs.github.com/en/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-code-scanning#using-queries-in-ql-packs
         # queries: security-extended,security-and-quality