**Summary**

Fixes #4320 and fixes #4323. We were using `fs.access` when selecting
the global prefix folder automatically which only checks for permissions
but not actual writeability. This caused issues on Heroku where one of
our first tries had the correct permissions but was on a read-only
file system.

**Test plan**

Existing cache folder fallback tests should be enough for now. We should
move the core of those tests for the newly added `fs.getFirstWriteableFolder`
method.

…ling