fix: lake: ltar caching bug with build -o

[tydeu]

This PR fixes a bug with Lake where caching an ltar produced via lake build -o would fail if restoreAllArtifacts was also true.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 1b63e7dfc6d95cbd608d700ccbac567f2c040ce2 --onto 61a3443a9569d548a235f785b9a33984bb7ff622. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-19 22:56:02)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 1b63e7dfc6d95cbd608d700ccbac567f2c040ce2 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-19 22:56:04)