ci: attest build only on push

When running the build for external PRs, we do not have access to
secrets (for good reason). By that, we also cannot perform the
attestation step. Just skip it in these cases.

Signed-off-by: Felix Moessbauer <[email protected]>

.github/workflows/build.yml

@@ -94,6 +94,7 @@ jobs:
 
       - name: attest extension artifacts
         uses: actions/attest-build-provenance@v1
+        if: github.event_name == 'push'
         with:
           subject-path: |
             build/Linux-Entra-SSO-v*