devshell: remove dependency on clan-cli

This stops the devshell from being re-built constantly, when the source code changes.

Based on a new feature in treefmt-nix: https://github.com/numtide/treefmt-nix/pull/208

checks/devshell/flake-module.nix

@@ -0,0 +1,22 @@
+{ ... }:
+{
+  perSystem =
+    { self', pkgs, ... }:
+    {
+      checks.devshell =
+        pkgs.runCommand "check-devshell-not-depends-on-clan-cli"
+          {
+            exportReferencesGraph = [
+              "graph"
+              self'.devShells.default
+            ];
+          }
+          ''
+            if grep -q "${self'.packages.clan-cli}" ./graph; then
+              echo "devshell depends on clan-cli, which is not allowed";
+              exit 1;
+            fi
+            mkdir $out
+          '';
+    };
+}