Improve CI

.github/workflows/ci.yml

@@ -10,4 +10,5 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
       - run: nix flake check

README.md

@@ -7,7 +7,6 @@ This is work-in-progress. I'll add documentation after it works
 reasonably well.
 
 <!-- `$  python3 -m ros2nix --help` -->
-
 ```
 usage: ros2nix [-h]
                [--output OUTPUT | --output-as-ros-pkg-name | --output-as-nix-pkg-name]

flake.nix

@@ -94,11 +94,15 @@
         packages = {
           default = ros2nix;
           inherit rosdistro rosdep-cache rosdep ros2nix;
+          inherit (pkgs) mdsh;
         };
         checks = {
-          mdsh = pkgs.runCommandNoCC "mdsh" {
-            nativeBuildInputs = [ ros2nix ];
-          } "cd ${self}; ${pkgs.mdsh}/bin/mdsh --frozen";
+          mdsh-check-readme = pkgs.runCommandNoCC "mdsh"
+            { nativeBuildInputs = [ ros2nix ]; } ''
+             mkdir $out; cd ${self};
+             if ! ${pkgs.mdsh}/bin/mdsh --frozen; then
+               echo 'Update README with `nix run .#mdsh`.'
+             fi'';
         };
       }
     );