Hide the "Check for Tor Browser Update..." menu entry when running under the sandbox.

Since I punted on full integration with torbutton in the sandboxed-tor-browser 0.0.3 release due to time constraints (and the relatively large patch that would be required), I opted for a simpler solution that will hopefully be acceptable for all torbutton branches.

If the TBB_SANDBOX env var is linux-v0, then the "Check for Tor Browser Update..." menu entry should be hidden.