We previously had two GOARCH variants for natlab testing
(gokrazy/natlabapp and gokrazy/natlabapp.arm64), but we didn't have
those variants for the "tsapp" (production, not testing) variant.
This adds arm64 for tsapp too. But there are two major users of arm64
for gokrazy: Raspberry Pis, and cloud VMs. So make two variants.
Updates #1866
Change-Id: Ib9efaf1255101e3cdbc31d49c76fd86dcba21bb1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>