Allow chromium buildtools path to be overriden in the environment.

This is useful in certain (admittedly unsupported) cases 
when trying to use tools from depot_tools outside of a 
chrome repository.  In this particular case, I was trying 
to "git cl format" something that wasn't a chrome 
repository.

BUG=0

Review URL: https://codereview.chromium.org/472553003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@289412 0039d316-1c4b-4281-b951-d872f2087c98

gclient_utils.py

@@ -658,6 +658,13 @@ def GetMacWinOrLinux():
 def GetBuildtoolsPath():
   """Returns the full path to the buildtools directory.
   This is based on the root of the checkout containing the current directory."""
+
+  # Overriding the build tools path by environment is highly unsupported and may
+  # break without warning.  Do not rely on this for anything important.
+  override = os.environ.get('CHROMIUM_BUILDTOOLS_PATH')
+  if override is not None:
+    return override
+
   gclient_root = FindGclientRoot(os.getcwd())
   if not gclient_root:
     # Some projects might not use .gclient. Try to see whether we're in a git