ci(gh-actions/update-flake-lock): Enable GPG commit signing

Author: PetarKirov

.github/workflows/update-flake-lock.yml

@@ -20,5 +20,8 @@ jobs:
       CACHIX_AUTH_TOKEN: ${{ secrets.CACHIX_AUTH_TOKEN }}
       CREATE_PR_APP_ID: ${{ secrets.NIX_FLAKE_UPDATE_PR_BOT_APP_ID }}
       CREATE_PR_APP_PRIVATE_KEY: ${{ secrets.NIX_FLAKE_UPDATE_PR_BOT_APP_PRIVATE_KEY }}
+      NIX_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      GIT_GPG_SIGNING_SECRET_KEY: ${{ secrets.GIT_GPG_SIGNING_SECRET_KEY }}
     with:
       runner: '["self-hosted", "Linux", "x86-64-v2"]'
+      sign-commits: true