ci: pin GitHub Actions to commit SHAs

Pin actions/checkout and actions/setup-go to their full commit SHAs
for supply-chain security, keeping the version tag as a comment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>

Author: DeepDiver1975

.github/workflows/ci.yml

@@ -11,10 +11,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Set up Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5
         with:
           go-version-file: go.mod