Explicitly disable the (required) jobs docker and setup_sh for

anything else but merge groups.

The idea comes from bevyengine/bevy#7824 (comment).

.github/workflows/test_installs.yml

@@ -1,18 +1,21 @@
 name: Test installations
 
 on:
+  pull_request:
   merge_group:
   workflow_dispatch:
   push:
     branches: [main]
 
 jobs:
   docker:
+    if: ${{ github.event_name == 'merge_group' }}
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
     - run: docker build -f .docker/Dockerfile . -t hacspec-v2
   setup_sh:
+    if: ${{ github.event_name == 'merge_group' }}
     strategy:
       matrix:
         os: