auto release lock on failure

**Is your feature request related to a problem? Please describe.**
According to [this discussion](https://forum.buildkite.community/t/buildkite-agent-lock-what-happens-on-failure/3358/3), locks are not automatically released if a job fails.

**Describe the solution you'd like**
Automatically release locks at exit

**Describe alternatives you've considered**
The suggested solutions (using pre-exit hooks) are somewhat cumbersome.

**Additional context**

Start by reviewing the Buildkite Agent's lock lifecycle and the job exit path, using the linked discussion for the reported failure behavior. Determine where failed jobs leave locks held and identify the existing tests around lock release or job termination. Done means locks are released automatically when a job exits, including after failure, with tests covering that behavior.

Written by the indexing model from the issue text.