Skip buildx on-success check for tagged builds

Tagged builds seem to erroneously fail the on-success check due to the
tests not having finished when the build begins. Since tagged builds are
only ever submitted once the tagged commit is confirmed to pass all
tests, this check will now be skipped.

.github/workflows/buildx.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Wait for tests to succeed
-        if: ${{ github.event.workflow_run.conclusion != 'success' }}
+        if: ${{ github.event.workflow_run.conclusion != 'success' && startsWith(github.ref, 'refs/tags') != true }}
         run: exit 1
       - name: checkout code
         uses: actions/checkout@v2