Out of date oleans in CI

Stream: mathlib4

[Yaël Dillies]

Bors failed at the "check for unused imports" step with the message

There are out of date oleans. Run `lake build` or `lake exe cache get` first

[Yaël Dillies]

... but that's what the previous steps do, right? Are we hitting some cache weirdness here? Should CI try lake build; lake exe shake if a lone lake exe shake fails?

[Damiano Testa]

I wonder if the issue is the --no-build flag that shake uses.