pythond and python3 are still downloaded from cipd, but python3 is no longer shim'd into
depot_tools on Linux, so is not available on PATH.
Modify PATH in vpython and vpython3 on Linux to point to the python3 download directory,
so that the version downloaded from cipd is used.

Bug: 1017812
Change-Id: I70bc8e7d460f60dcbfa5e141127edecfc2b44744
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1893982
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>