build: Do not run release-ghcr on pull_request

The `pull_request` trigger tries to merge the base branch, and
Hydra uses the commit tip, so nix will usually not resolve the
correct artifact.

.github/workflows/release-ghcr.yml

@@ -5,7 +5,6 @@ on:
     types:
       - published
   push: {}
-  pull_request: {}
   workflow_dispatch: {}
   schedule:
     # Everyday at 4:00 AM UTC