bazel: distinguish darwin patches from generic patches

The '' + '' somewhere in the middle is easily missed and extremely
inelegant.

Also removes unnecessary `stdenv` qualifiers from calls to `lib`.

pkgs/development/tools/build-managers/bazel/default.nix

@@ -9,7 +9,7 @@
 }:
 
 let
-  srcDeps = stdenv.lib.singleton (
+  srcDeps = lib.singleton (
     fetchurl {
       url = "https://github.com/google/desugar_jdk_libs/archive/f5e6d80c6b4ec6b0a46603f72b015d45cf3c11cd.zip";
       sha256 = "c80f3f3d442d8a6ca7adc83f90ecd638c3864087fdd6787ffac070b6f1cc8f9b";
@@ -28,7 +28,7 @@ stdenv.mkDerivation rec {
 
   version = "0.15.2";
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     homepage = "https://github.com/bazelbuild/bazel/";
     description = "Build tool that builds code quickly and reliably";
     license = licenses.asl20;
@@ -74,7 +74,8 @@ stdenv.mkDerivation rec {
     }
   '';
 
-  postPatch = stdenv.lib.optionalString stdenv.hostPlatform.isDarwin ''
+  postPatch = let
+    darwinPatches = ''
       # Disable Bazel's Xcode toolchain detection which would configure compilers
       # and linkers from Xcode instead of from PATH
       export BAZEL_USE_CPP_ONLY_TOOLCHAIN=1
@@ -103,7 +104,8 @@ stdenv.mkDerivation rec {
       for wrapper in "''${wrappers[@]}"; do
         sed -i -e "s,/usr/bin/install_name_tool,${cctools}/bin/install_name_tool,g" $wrapper
       done
-  '' + ''
+    '';
+    genericPatches = ''
       find src/main/java/com/google/devtools -type f -print0 | while IFS="" read -r -d "" path; do
         substituteInPlace "$path" \
           --replace /bin/bash ${customBash}/bin/bash \
@@ -139,6 +141,8 @@ stdenv.mkDerivation rec {
 
       patchShebangs .
     '';
+    in lib.optionalString stdenv.hostPlatform.isDarwin darwinPatches
+     + genericPatches;
 
   buildInputs = [
     jdk