Allow Opting Out of Immutable Flags for Yarn 2

[jpulec]

In some cases, a user may not want the --immutable-cache or --immutable flag to be passed. One example I have is when you have a repo with some workspace packages that you're not passing to Heroku by adding them to your .slugignore. In this case, the lock files would change, which means calling install --immutable will fail.

Additionally, when yarn install --immutable does fail for this reason, there's no indication, and the build just appears to hang.

[danielleadams]

#836 solves half of this, but we still need a way to call Yarn 2 without --immutable. @jpulec how should the buildpack detect that yarn.lock should be mutable?

[jpulec]

I'm wondering if it makes sense to allow disabling this via an environment variable. Something that is off by default, but could be set to allow lockfile discrepencies, i.e. YARN_IMMUTABLE=false.

[joeyfigaro]

Any updates here?