Don't push to pkg.github.com on pull_request.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

.github/workflows/build-docker-image-run-drc-for-cell-gds-using-magic.yml

diff --git a/.github/workflows/build-docker-image-run-drc-for-cell-gds-using-magic.yml b/.github/workflows/build-docker-image-run-drc-for-cell-gds-using-magic.yml
index 4b1c052..6b9a8f1 100644
--- a/.github/workflows/build-docker-image-run-drc-for-cell-gds-using-magic.yml
+++ b/.github/workflows/build-docker-image-run-drc-for-cell-gds-using-magic.yml
@@ -52,7 +52,7 @@
         tag_with_ref: true
         tag_with_sha: true
         add_git_labels: true
-        push: true
+        push: ${{ startsWith(github.ref, 'refs/heads/') }}
 
 
   build-docker-image: