Two bug fixes:
1. when tailscale update is executed as root, `os.UserCacheDir` may
return an error because `$XDG_CACHE_HOME` and `$HOME` are not set;
fallback to `os.TempDir` in those cases
2. on some weird distros (like my EndeavourOS), `/usr/sbin` is just a
symlink to `/usr/bin`; when we resolve `tailscale` binary path from
`tailscaled`, allow `tailscaled` to be in either directory
Updates #755
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>
77127a2494