fix: lake: ill-formed build output handling

[tydeu]

This PR makes Lake no longer error if build outputs found in a trace file (or in the artifact cache) are ill-formed.

This is caused a problem with the CI cache and is just generally too strict.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2025-09-26 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2025-09-27 01:29:44)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 5b8d4d72104e66de541339027b41b7e434c1ab7e --onto ac0b82933f6eac9914011ca2caf38d0e4e991160. You can force Mathlib CI using the force-mathlib-ci label. (2025-09-27 03:02:44)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2025-09-26 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2025-09-27 01:29:45)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 5b8d4d72104e66de541339027b41b7e434c1ab7e --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-09-27 03:02:46)