Semgrep action pinning rules.yaml checkout

Author: eloymg

.github/workflows/semgrep.yaml

@@ -21,7 +21,9 @@ jobs:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           repository: grafana/security-github-actions
-          ref: ${{ github.repository == 'grafana/security-github-actions' && (github.head_ref || github.ref_name) || '' }}
+          ref: ${{ github.repository == 'grafana/security-github-actions' &&
+            (github.head_ref || github.ref_name) ||
+            '8b10f82433323c48383277b3abc8c87d26564e63' }}
           sparse-checkout: |
             semgrep/custom-rules.yaml
             semgrep/format-results.sh