ci: add nix code formatting check

.gitlab-ci.yml

@@ -1,3 +1,7 @@
 test:
   script:
     - nix-build --no-out-link --show-trace tests
+
+nixpkgs-fmt:
+  script:
+    - ./check-fmt

check-fmt

@@ -0,0 +1,10 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash
+#
+# Just because the nixpkgs-fmt error message is not super readable. Used by
+# CI.
+
+if ! nixpkgs-fmt --check . ; then
+  echo 'run `nixpkgs-fmt .` to fix it'
+  exit 1
+fi

shell.nix

@@ -6,5 +6,6 @@ in
 pkgs.mkShell {
   buildInputs = [
     poetry
+    pkgs.nixpkgs-fmt
   ];
 }