Draft: hadrian: Persist the used flavour name in the build root

This executes the idea proposed in #20650, teaching Hadrian to persist the flavour used to build a build root. This both makes use of multiple build roots more ergonomic (as the user no longer needs to repeat themselves) and avoids a tiresome foot-gun (forgetting to specify a flavour, often wholly invalidating a build root).

Closes #20650.

Incidentally, this required dropping the withoutActions from Hadrian's main entrypoint. However, I believe this is a semantics-preserving change since there are currently no occurrences of wants or action in the build system. Moreover, I think that dropping actions is a potential source of future surprises.