Retry Node install without NVM mirror after diagnostics

[sjchmiela]

Summary

• log nvm ls-remote output whenever a custom Node.js install fails so diagnostics are always available
• refactor the worker installNode flow to share an install helper, rerun without NVM_NODEJS_ORG_MIRROR when present, and keep downstream behavior unchanged
• add unit tests that cover diagnostics logging, the mirror-free retry, and eventual failure when both attempts fail

Testing

• Not run (not requested)

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