Add actions job to notice outdated dependencies

Author: dtolnay

.github/workflows/ci.yml

@@ -20,3 +20,12 @@ jobs:
           toolchain: ${{matrix.rust}}
       - run: cargo test
         working-directory: _skeptic
+
+  outdated:
+    name: Outdated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+      - uses: dtolnay/install@cargo-outdated
+      - run: cargo outdated --exit-code 1