`minicore`: use the host's `core` when available

[tgross35]

From discussion at https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Using.20.60minicore.60.20for.20more.20compiletests, it would be ideal to replace minicore with the host's core if that is built. @jieyouxu pointed out that this did exist for the ABI compatibility test at some point adb6d47#diff-55f122dec36f78625c830ef254174f8ac89b16959557e11dff4d33ef9fd12438L70.

The main advantage here is it gives us a better chance of catching (unlikely) deviations between core and minicore, as opposed to always using minicore.