tests: actually execute vars checks in CI

lib/build-clan/interface.nix

@@ -34,7 +34,7 @@ in
         else if v == null then
           throw "Please set either clan.self or clan.directory"
         else
-          "${v}"
+          v
       ) lib.types.path;
       default = builtins.toString self;
       defaultText = "Root directory of the flake";