In the sandboxed app from the app store, we cannot check
`/Library/Preferences/com.apple.commerce.plist` or run `softwareupdate`.
We can at most print a helpful message and open the app store page.
Also, reenable macsys update function to mark it as supporting c2n
updates. macsys support in `tailscale update` was fixed.
Updates #755
Signed-off-by: Andrew Lytvynov <awly@tailscale.com>