Cache default ELM_HOME location

[wkaluza]

Netlify provides an Elm compiler installation, but does not cache the Elm modules which are downloaded during deploys. This leads to unnecessary repeated downloads which could be cached.

There have been several attempts by members of the Elm community to enable caching in order to improve build performance. The solutions tend to be brittle and rely on build script implementation details like certain environment variables being set and available to user code.

• https://discourse.elm-lang.org/t/caching-elm-downloads-on-netlify-builds/5077 - this solution has been reported as no longer working, potentially due to the NETLIFY_CACHE_DIR env variable no longer being visible to user code.
• https://discourse.elm-lang.org/t/caching-elm-downloads-on-netlify-builds-reprise/5923 - this puts the burden of setting ELM_HOME on the user, completely reinstalling any Elm modules present. This particular example also has problems, as caching the whole local elm-stuff/ is not without risk according to the creator of Elm: Point to "how to make CI fast" link when cache is messed up elm/compiler#1845 (comment)
• Some attempts out there, like https://github.com/JoelQ/elm-netlify-minimal/blob/master/netlify.toml are simply wrong, in addition to relying on the location of the Netlify cache directory staying where it is.

This has clearly been bugging people, and there are multiple non-solutions littering the Internet already. Elm is perhaps not a major player in the frontend world, but if the compiler is available on Netlify, perhaps caching Elm downloads by default would also be in order? If this is deemed a problem worth solving, I see two solutions:

1. Set the ELM_HOME env variable in the Netlify build scripts to point to $NETLIFY_CACHE_DIR/.elm. Downloads will be saved there directly and will persist between deploys.
2. Leave ELM_HOME unset and cache the default $HOME/.elm location using existing infrastructure.

I have a changeset which uses method 2, as that seems to be in line with current practice. Tested and confirmed to work locally using the test container provided. If maintainers approve, I am happy to submit a pull request.

[wkaluza]

[ping] Would there be any interest in this? Shall I just submit the PR?