impure-checks: unset CLAN_DIR

Unset `CLAN_DIR` in the impure checks, so that it won't reference the
users configuration.

Fixes #1419

checks/impure/flake-module.nix

@@ -7,6 +7,8 @@
         #!${pkgs.bash}/bin/bash
         set -euo pipefail
 
+        unset CLAN_DIR
+
         export PATH="${
           lib.makeBinPath [
             pkgs.gitMinimal