Do not clean the sbt bridge cache on every bootstrapped compile

[smarter]

Previously, any compilation of a bootstrapped project removed the sbt
bridge cache which then had to be recompiled. This is too extreme: we
should only do it when the bridge needs to be recompiled.

[Duhemm]

LGTM, if I understand correctly:

• Doing a bootstrapped compile always packages the sources of bridge so that sbt can then compile it.
• This used to clean the bridge each time.
• Now we only clean the bridge when we explicitly want to compile it.

[smarter]

Yes, exactly :).