`nvm_installation_dir` mutated outside of job

As part of the `pre-exit` hook, the `nvm_installation_dir` is [fetched from the BuildKite agent key/value store](https://github.com/Automattic/nvm-buildkite-plugin/blob/561d5d72bbb89b826f2f4083e20084f6b89542a3/hooks/pre-exit#L5).

If I understand [the docs](https://buildkite.com/docs/agent/v3/cli-meta-data) correctly, this store is global to all agents. This means that jobs could mutate `nvm_installation_dir` and [affect other jobs](https://github.com/Automattic/nvm-buildkite-plugin/blob/561d5d72bbb89b826f2f4083e20084f6b89542a3/hooks/pre-exit#L7).

Example:
- [Job A](https://buildkite.com/automattic/gutenberg-mobile/builds/7737#018b8668-77a8-49f4-bf9b-f3136375d1b4) generated `w6i8GGXW`
- [Job B](https://buildkite.com/automattic/gutenberg-mobile/builds/7737#018b8668-70f4-4266-a828-2e43853d64bf) generated `riEsqXM4`
- [Job A](https://buildkite.com/automattic/gutenberg-mobile/builds/7737#018b8668-77a8-49f4-bf9b-f3136375d1b4) looked for `riEsqXM4` at cleanup, when we would expect it to use `w6i8GGXW`

Start by reading hooks/pre-exit and the linked Buildkite metadata documentation to verify how nvm_installation_dir is stored and retrieved. Trace the cleanup path for concurrent jobs, then confirm that each job uses its own installation directory without affecting another job's metadata.

Written by the indexing model from the issue text.