Since the incubator parses the UID in base 0 via flags.IntVar()
and isRootUser() uses base 10, it was possible to get inconsistent
results between the two using non-base 10 numbers.
Updates tailscale/corp#50380
Change-Id: I9b2fb16bc37153df7b695533cc73a3499cb774eb
Signed-off-by: Francois Marier <francois@tailscale.com>