Add a CI step to check that there are no changed files after Clippy

This will catch things like Cargo.lock being out of date with changes to
Cargo.toml, etc.

.github/workflows/ci.yml

@@ -24,6 +24,9 @@ jobs:
       - name: Clippy
         run: nix develop --command cargo clippy --all-targets
 
+      - name: Repository cleanliness
+        run: git diff --exit-code
+
       - name: Test
         run: nix build -L --no-sandbox .#test