fix(ci.formatting): copy source to writable dir before nixfmt-tree on macOS

Author: TrustworthyAdult

ci/checks/formatting.nix

@@ -5,8 +5,10 @@
 }:
 
 pkgs.runCommand "formatting-check" { } ''
+  cp -r ${../../.} ./src
+  chmod -R u+w ./src
   ${
     pkgs.lib.getExe self.formatter.${pkgs.stdenv.hostPlatform.system}
-  } --no-cache --fail-on-change ${../../.}
+  } --no-cache --fail-on-change ./src
   touch $out
 ''