Turn nailgun initialization message down to debug

[huonw]

I think the intention with these log lines is logging expensive operations. However, the NailgunPool::new operation itself isn't expensive, and so doesn't need to be logged (sometimes twice) on every start-up.

As a fairly-expert Pants user, I'm not sure what information one should get from knowing that there's potentially 16 processes managed by nailgun, and other less-expert users likely have even less use for it!

Fixes #21863 , revisits #20946 (comment) (particularly the last paragraph).

[benjyw]

Yeah, this is good, since those messages give the impression that the nailgun startup is expensive, but actually it's the solver that is taking the time that those messages seem to be hanging on...