add formatting job to Nix CI and define fmt shell in flake.nix

Author: b1llow

.github/workflows/nix.yml

@@ -83,4 +83,4 @@ jobs:
           nix fmt -- --ci .
       - name: Check cargo formatting
         run: |
-          nix develop . -c cargo fmt --check
+          nix develop .#fmt -c cargo fmt --check

flake.nix

@@ -89,6 +89,11 @@
               RUST_SRC_PATH = "${rustPlatform.rustLibSrc}";
             }
           );
+          fmt = mkShell {
+            packages = [
+              rustPlatform.rust.cargo
+            ];
+          };
         };
 
       }