spectrwm: make build src name independent

(cherry picked from commit 7865091786a037a9545d181e31afa7b4d8fd0be2)

pkgs/applications/window-managers/spectrwm/default.nix

···
     xcbutilwm
   ];
 
-  sourceRoot = let
+  prePatch = let
     subdir = if stdenv.isDarwin then "osx" else "linux";
-  in "source/${subdir}";
+  in "cd ${subdir}";
 
   makeFlags = [ "PREFIX=${placeholder "out"}" ];