chromium: don't use pkgsBuildTarget

I'm pretty sure this was a mistake — in Nixpkgs the target platform is
the platform that the program being built should output executables
for — i.e., it's only relevant for a compiler, which Chromium is not.

Tested that cross-compilation of Electron still works.

(cherry picked from commit a269b980e9)

pkgs/applications/networking/browsers/chromium/common.nix

@@ -3,7 +3,6 @@
 , buildPackages
 , buildPlatform
 , pkgsBuildBuild
-, pkgsBuildTarget
 # Channel data:
 , channel, upstream-info
 # Helper functions:
@@ -384,8 +383,8 @@ let
     llvmCcAndBintools = symlinkJoin {
       name = "llvmCcAndBintools";
       paths = [
-        pkgsBuildTarget.${llvmPackages_attrName}.llvm
-        pkgsBuildTarget.${llvmPackages_attrName}.stdenv.cc
+        buildPackages.${llvmPackages_attrName}.llvm
+        buildPackages.${llvmPackages_attrName}.stdenv.cc
       ];
     };