Retry failed apt install during bootstrap

[johnsca]

Occasionally, apt would not be able to acquire the lock and would fail, only to succeed on the next attempt.

Fixes #91

[marcoceppi]

LGTM 👍

[mbruzek]

Is it just me or is this else not at the right indentation level?

[johnsca]

I answered this via other channels, but for posterity:

The else here is on the try / except block, and will be executed if there are no exceptions raised. It is almost the same thing as putting the break inside the try block except that exceptions raised in the else block are not caught by any except blocks (though in this case, break can't raise a CalledProcessError anyway).