ci: re‐enable flake checker fail mode

Looks like this should be fine now that Nixpkgs has been updated and
the static build isn’t used any more.

.github/workflows/check-and-test.yaml

@@ -21,10 +21,8 @@ jobs:
 
       - name: Check health of flake.lock
         uses: DeterminateSystems/flake-checker-action@main
-        # TODO: re-enable fail mode when we find a way to bump Nixpkgs to 24.05
-        # without breaking the static Rust build
-        #with:
-        #  fail-mode: true
+        with:
+          fail-mode: true
 
       - name: Check Rust formatting
         run: nix develop --command cargo fmt --check