set pr sh from context

.github/workflows/test-and-push.yml

@@ -12,6 +12,11 @@ jobs:
   test-and-push:
     runs-on: ubuntu-latest
     steps:
+      - name: Set Pull Request Head SHA
+        if: ${{ github.event_name == 'pull_request' }}
+        run: |
+          long_sha=${{ github.event.pull_request.head.sha }}
+          echo "PR_HEAD_SHA=${long_sha:0:7}" >> $GITHUB_ENV
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:

docker/common

@@ -1,11 +1,10 @@
 REGISTRY=ghcr.io
 PACKAGE_ORG=mrc-ide
 PACKAGE_NAME=grout
-if [[ "${GITHUB_EVENT_NAME}" = pull_request ]]; then
-  LONG_SHA=${GITHUB_SHA}
-  GIT_SHA=${LONG_SHA:0:7}
-else
+if [[ -z "${PR_HEAD_SHA}" ]]; then
   GIT_SHA=$(git -C "$PACKAGE_ROOT" rev-parse --short=7 HEAD)
+else
+  GIT_SHA=${PR_HEAD_SHA}
 fi
 if [[ -v "BRANCH_NAME" ]]; then
     GIT_BRANCH=${BRANCH_NAME}