github actions: Actually build the right commit

.github/workflows/main.yml

@@ -40,6 +40,12 @@ jobs:
         uses: actions/checkout@v2
         with:
           submodules: recursive
+          # By default, for PR builds, Actions will check out a merge commit between the actual
+          # PR branch and the base branch (normally master). This isn't what we want as it means
+          # the actually pushed commit isn't getting built, so it can't then be used without being
+          # merged first. This makes testing much more difficult.
+          # This option makes it actually check out the PR's commit instead.
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Log into ghcr.io
         uses: docker/login-action@v1