nvm_installation_dir mutated outside of job

[twstokes]

As part of the pre-exit hook, the nvm_installation_dir is fetched from the BuildKite agent key/value store.

If I understand the docs correctly, this store is global to all agents. This means that jobs could mutate nvm_installation_dir and affect other jobs.

Example:

• Job A generated w6i8GGXW
• Job B generated riEsqXM4
• Job A looked for riEsqXM4 at cleanup, when we would expect it to use w6i8GGXW