Update build.yml

Author: ntkme

.github/workflows/build.yml

@@ -175,6 +175,7 @@ jobs:
           tar -czf dartsdk-linux-${{ matrix.target-arch }}-release.tar.gz -C dart-sdk/sdk/out/Release* -- dart-sdk
 
       - name: Generate artifact attestation
+        if: startsWith(github.ref, 'refs/tags/')
         uses: actions/attest-build-provenance@v2
         with:
           subject-path: dartsdk-linux-${{ matrix.target-arch }}-release.tar.gz