Configure pre-commit-hooks via shell.nix

This configures a git pre-commit hook as described in [1]. For
now this only enables `stylish-haskell`.

The way this works is that the code added to `shellHook` generates
a `.pre-commit-config.yaml` and configures the hook with git.

PS: If you are using some `direnv` based solution such as lorri
you might want to add `eval $shellHook` to your `.envrc` file.

[1] https://github.com/cachix/pre-commit-hooks.nix#getting-started

shell.nix

@@ -6,6 +6,13 @@ let
   pyEnv = pkgs.python3.withPackages (ps: [ packageSet.sphinxcontrib-haddock.sphinxcontrib-domaintools ps.sphinx ps.sphinx_rtd_theme ]);
   # Called from Cabal to generate the Haskell source for the metatheory package
   agdaWithStdlib = agdaPackages.agda.withPackages [ agdaPackages.standard-library ];
+  # Configure project pre-commit hooks
+  pre-commit-check = pkgs.nix-pre-commit-hooks.run {
+    src = ./.;
+    hooks = {
+      stylish-haskell.enable = true;
+    };
+  };
 in haskell.packages.shellFor {
   nativeBuildInputs = [
     # From nixpkgs
@@ -58,13 +65,15 @@ in haskell.packages.shellFor {
   # we have a local passwords store that we use for deployments etc.
   PASSWORD_STORE_DIR = toString ./. + "/secrets";
 
-  shellHook = 
+  shellHook = ''
+    ${pre-commit-check.shellHook}
+  ''
     # Work around https://github.com/NixOS/nix/issues/3345, which makes
     # tests etc. run single-threaded in a nix-shell.
     # Sets the affinity to cores 0-1000 for $$ (current PID in bash)
     # Only necessary for linux - darwin doesn't even expose thread
     # affinity APIs!
-    pkgs.lib.optionalString pkgs.stdenv.isLinux ''
-      taskset -pc 0-1000 $$ 
+   + pkgs.lib.optionalString pkgs.stdenv.isLinux ''
+      ${pkgs.utillinux}/bin/taskset -pc 0-1000 $$
     '';
 }