Debian is deleting /usr/bin/python by default, so re-use the existing
python3 interp that our wrapper was invoked with.

Bug: 1182059
Test: `./repo` still works w/out /usr/bin/python
Change-Id: I1806f2ae2f8c6fe8b5f5a95fd2e50735d6e3196a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2715994Reviewed-by: Michael Mortensen <mmortensen@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>