improve CI times by pre-building stylish-haskell on hydra

.buildkite/pipeline.yml

@@ -21,12 +21,12 @@ steps:
       system: x86_64-linux
 
   - label: 'Check Stylish Haskell'
-    command: 'nix-shell --run .buildkite/check-stylish.sh'
+    command: 'nix-shell -A simple-ci-shell --run .buildkite/check-stylish.sh'
     agents:
       system: x86_64-linux
 
   - label: 'HLint'
-    command: 'nix-shell --run "hlint lib"'
+    command: 'nix-shell -A simple-ci-shell --run "hlint lib"'
     agents:
       system: x86_64-linux
 

release.nix

@@ -25,6 +25,12 @@ let
     "${mingwW64.config}" = mapTestOnCross mingwW64 (packagePlatformsCross project);
   }
   // {
+    ci-tools = {
+      inherit (import ./nix/iohk-common.nix {}) hlint;
+      inherit ((import ./nix/nixpkgs-haskell.nix {}).haskellPackages) stylish-haskell;
+    };
+    inherit ((import ./. {}).pkgs.haskell-nix) haskellNixRoots;
+
     # This aggregate job is what IOHK Hydra uses to update
     # the CI status in GitHub.
     required = mkRequiredJob (

shell.nix

@@ -1 +1,8 @@
-(import ./default.nix {}).shell
+let
+  self = import ./default.nix {};
+  # a shell that only has basic CI tools, and no dependencies
+  simple-ci-shell = self.pkgs.stdenv.mkDerivation {
+    name = "simple-ci-shell";
+    buildInputs = [ self.pkgs.haskellPackages.stylish-haskell self.iohkLib.hlint ];
+  };
+in self.shell // { inherit simple-ci-shell; }