Update deprecated actions/checkout v2 -> v4 (#3)

- actions/checkout v2 -> v4

Found by an org-wide actionlint sweep; the lint tooling itself is staying in the core repos only, but these fixes stand on their own.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mattgodbolt

.github/workflows/build-test.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: windows-2022
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Add msbuild to PATH
         uses: microsoft/setup-msbuild@v1.1