ci: pin actions to commit SHAs to prevent RCE

Pin actions/checkout and actions/setup-go to their full commit
SHAs instead of mutable tags, per review feedback.

- actions/checkout@v4 → 34e114876b0b11c390a56381ad16ebd13914f8d5
- actions/setup-go@v5 → 40f1582b2485089dde7abd97c1529aa768e1baff

.gitea/workflows/check.yml

@@ -10,9 +10,9 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
-      - uses: actions/setup-go@v5
+      - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5
         with:
           go-version-file: go.mod