Add actions job to notice outdated dependencies

Author: dtolnay

.github/workflows/ci.yml

@@ -40,3 +40,12 @@ jobs:
       - run: git rebase HEAD FETCH_HEAD
       - run: cargo test
         working-directory: ${{matrix.project}}
+
+  outdated:
+    name: Outdated
+    runs-on: ubuntu-latest
+    if: github.event_name != 'pull_request'
+    steps:
+      - uses: actions/checkout@v3
+      - uses: dtolnay/install@cargo-outdated
+      - run: cargo outdated --workspace --exit-code 1