ci: pin GitHub Actions to commit SHAs

Pin actions/checkout, setup-node, upload-artifact, setup-dotnet, and
github-script to immutable commit SHAs (version kept in a trailing comment) so
a moved tag cannot change the code executed in CI.

Refs Heretek-Games/drop#69 (audit finding F-23).

.github/workflows/ci.yml

@@ -11,10 +11,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # actions/checkout@v4
 
       - name: Setup Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # actions/setup-node@v4
         with:
           node-version: 22
           cache: npm