ci: only run bulk units if stuff has materially changed

Author: chingor13

.github/workflows/ci.yaml

@@ -20,8 +20,30 @@ on:
   pull_request:
 name: ci
 jobs:
+  bulk-filter:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: read
+    # Set job outputs to values from filter step
+    outputs:
+      src: ${{ steps.filter.outputs.src }}
+      ci: ${{ steps.filter.outputs.ci }}
+    steps:
+    - uses: actions/checkout@v4
+    - uses: dorny/paths-filter@v3
+      id: filter
+      with:
+        filters: |
+          src:
+            - '**/*.java'
+            - '**/pom.xml'
+          ci:
+            - '.github/workflows/**'
+            - '.kokoro/**'
   units:
     runs-on: ubuntu-latest
+    needs: bulk-filter
+    if: ${{ needs.bulk-filter.outputs.src == 'true' }}
     strategy:
       fail-fast: false
       matrix: