CI: use GitHub API to exempt dependabot from changelog requirement (#378)

* CI: use GitHub API to exempt dependabot from changelog requirement

* Write changelog

.github/workflows/require-changelog-for-PRs.yml

@@ -4,28 +4,34 @@ on:
   pull_request:
 
 jobs:
+  get-submitter:
+    name: Get the username of the PR submitter
+    runs-on: ubuntu-latest
+    outputs:
+      submitter: ${{ steps.get-submitter.outputs.submitter }}
+    steps:
+      # cannot use `github.actor`: the triggering commit may be authored by a maintainer
+      - name: Get PR submitter
+        id: get-submitter
+        run: curl -sSfL https://api.github.com/repos/imsnif/bandwhich/pulls/${PR_NUMBER} | jq -r '"submitter=" + .user.login' | tee -a $GITHUB_OUTPUT  
+
   check-changelog:
     name: Check for changelog entry
-    runs-on: ubuntu-latest
-    # allow dependabot PRs to have no changelog
-    if: github.actor != 'dependabot[bot]'
+    needs: get-submitter
     env:
       PR_NUMBER: ${{ github.event.number }}
+      PR_SUBMITTER: ${{ needs.get-submitter.outputs.submitter }}
       PR_BASE: ${{ github.base_ref }}
+    runs-on: ubuntu-latest
+    # allow dependabot PRs to have no changelog
+    if: ${{ needs.get-submitter.outputs.submitter != 'dependabot[bot]' }}
     steps:
       - uses: actions/checkout@v4
 
       - name: Fetch PR base
         run: git fetch --no-tags --prune --depth=1 origin
 
-      # cannot use `github.actor`: the triggering commit may be authored by a maintainer
-      - name: Get PR submitter
-        id: get-submitter
-        run: curl -sSfL https://api.github.com/repos/imsnif/bandwhich/pulls/${PR_NUMBER} | jq -r '"submitter=" + .user.login' | tee -a $GITHUB_OUTPUT
-
       - name: Search for added line in changelog
-        env:
-          PR_SUBMITTER: ${{ steps.get-submitter.outputs.submitter }}
         run: |
           ADDED=$(git diff -U0 "origin/${PR_BASE}" HEAD -- CHANGELOG.md | grep -P '^\+[^\+].+$')
           echo "Added lines in CHANGELOG.md:"