lake exe cache get in CI and Mathlib.Init

Stream: mathlib4

[Damiano Testa]

A while back, the CI step that checked whether to try to download the cache or simply go for a full re-build from scratch broke:

#general>`lake build --no-build` at `v4.13.0-rc3` is broken?

As a consequence, the check was removed and lake would try to find the cache also in situations where there was no hope to find anything.

It seems that now the check is working again (and has been for some time), so I reinstated the old CI step.

Keep an eye out for strange build failures, since this might be a cause, as it is in the main CI build step.

[Damiano Testa]

For reference, #18715 is the PR that re-introduces the check.