".dirty" in firmware version does not appear reliably

[sbourdeauducq]

The version string is cached by cargo and recomputed when the contents of .git changes. This leads to inconsistencies around .dirty, which depends on the whole repository contents and not only .git.

Can be reproduced by compiling the firmware from a pristine git tree, then making a modification to the source, and recompiling without deleting the previous compilation results.

[whitequark]

Fixed. But this makes builds take 8 seconds regardless of whether they're fresh or not because of some issue with dependency tracking I never figured out.