nixpkgs: don't fail build if a patch has already been applied

pkgs/additional/nixpkgs/default.nix

@@ -51,6 +51,13 @@ let
       inherit variant;
       date = unpatchedSrc.lastModifiedDate;
     };
+    # skip applied patches
+    prePatch = ''
+      realpatch=$(command -v patch)
+      patch() {
+         OUT=$($realpatch "$@") || echo "$OUT" | grep "Skipping patch" -q
+      }
+    '';
   };
 
   src = if doPatch then patchedSrc else { outPath = unpatchedSrc; };