fetchpatch: allow callers to specify postFetch sensibly

Before this fix, it seemed to be trying to merge our postFetch with the
patch normalization logic, but accidentally clobbering the whole thing
with the passed-in value.

pkgs/build-support/fetchpatch/default.nix

@@ -27,4 +27,4 @@ fetchurl ({
       "$tmpfile" > "$out"
     ${args.postFetch or ""}
   '';
-} // builtins.removeAttrs args ["stripLen" "addPrefixes" "excludes"])
+} // builtins.removeAttrs args ["stripLen" "addPrefixes" "excludes" "postFetch"])