feat(yarn): ignore yarn-path if binary does not exist

[dschaller]

Changes:

This PR changes the behavior of the checkYarnrc function to validate that the binary pointed to by yarn-path exists before returning the path. If it does not exist the yarn-path is set to null.

Context:

If a repository does not include a yarn binary in its source code but does set a yarn-path (which can be used after running initialization scripts) the current behavior of this function will return the path which will not exist when renovate runs updates. This changes ensures that the path is valid, by ensuring the binary exists, before running updates.

See #12321 for more information

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[CLAassistant]

All committers have signed the CLA.

[rarkins]

Have you tested this? I'm not sure this achieves what you expect. I think the new .yarnrc needs to be written to disk and not sure that's happening automatically.

[dschaller]

@rarkins pushed an update that's been tested here

[dschaller]

@rarkins thanks for the review! Pushed an update when you have a minute :)

[renovate-release]

🎉 This PR is included in version 28.12.0 🎉

The release is available on:

• GitHub release
• 28.12.0

Your semantic-release bot 📦🚀

[busches]

Shouldn't this error or at least warn the user a setting they set is being ignored?

[rarkins]

@busches sure, I'd accept a PR which includes a log warning if so