buildMozillaMach: move env variables into env (#487963)

pkgs/build-support/build-mozilla-mach/default.nix

@@ -367,10 +367,6 @@ buildStdenv.mkDerivation {
     "-l"
   ];
 
-  # if not explicitly set, wrong cc from buildStdenv would be used
-  HOST_CC = "${llvmPackagesBuildBuild.stdenv.cc}/bin/cc";
-  HOST_CXX = "${llvmPackagesBuildBuild.stdenv.cc}/bin/c++";
-
   nativeBuildInputs = [
     autoconf
     cargo
@@ -643,7 +639,12 @@ buildStdenv.mkDerivation {
   makeFlags = extraMakeFlags;
   separateDebugInfo = enableDebugSymbols;
   enableParallelBuilding = true;
-  env = lib.optionalAttrs stdenv.hostPlatform.isMusl {
+  env = {
+    # if not explicitly set, wrong cc from buildStdenv would be used
+    HOST_CC = "${llvmPackagesBuildBuild.stdenv.cc}/bin/cc";
+    HOST_CXX = "${llvmPackagesBuildBuild.stdenv.cc}/bin/c++";
+  }
+  // lib.optionalAttrs stdenv.hostPlatform.isMusl {
     # Firefox relies on nonstandard behavior of the glibc dynamic linker. It re-uses
     # previously loaded libraries even though they are not in the rpath of the newly loaded binary.
     # On musl we have to explicitly set the rpath to include these libraries.