main: improve launcher update messaging wrt system installs

Some users get repo from their distro (e.g. /usr/bin/repo), so the
suggestion to copy over top of it makes people uneasy, if it's even
possible in the first place.

Bug: https://crbug.com/gerrit/12335
Change-Id: I9a0c83d6ba0f466fa8e6d61f674ee13396f9a968
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256893


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>