ci: disallow continue on check failure

Has been hiding issues with the NUR update for a month due to a change
in `buildRustPackage`.

.github/workflows/build.yml

@@ -19,7 +19,6 @@ jobs:
       - name: Check ${{ matrix.check }}
         # Depends on nixos/nix#7759 to simply `nix flake check`
         run: nix run .#checks.$(nix eval --raw --impure --expr "builtins.currentSystem").${{ matrix.check }}
-        continue-on-error: true
 
   build-and-update:
     strategy: