build(`xtask`): reduce dependencies in non-CI usecases

[fogti]

This puts the ci subcommand behind a feature flag (enabled by default for backward compatibility). In particular, this should make it possible to massively reduce the amount of dependencies which have to be built in order to build the hermit kernel as part of the hermit build script (previously 101, now 33, for xtask).

This also gets rid of the flate2 dependency entirely.

[fogti]

(this would also subsequently massively reduce my concerns about the kernel xtask package not being a library which can be linked into build scripts)