update-manpages: force use of active interp

Since the repo wrapper uses #!/usr/bin/python, use the python3 that
this wrapper is actively using.

Change-Id: I03d1e54418d18a504eec628e549b4cc233621c45
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/345294


Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Mike Frysinger <vapier@google.com>