Make cache directory configurable

[UiP9AV6Y]

Pull Request

Read the cache directory from the environment, falling back to
a location under N_PREFIX

Problem

fixes #715

Solution

Initialize the cache directory variable in a similar fashion
as N_PREFIX

[shadowspawn]

Nice work adding a test to go with the feature.

I think this would be tidier with N_CACHE_PREFIX matching N_PREFIX, and the n/versions getting added onto that as before. Exposing the internal CACHE_DIR directly has exported a directory from deeper in the n controlled folder, so for example can't add another folder beside "versions" in future. The internal CACHE_DIR can then be constructed much as before.

Simple story:

• N_PREFIX defaults to /usr/local
• N_CACHE_PREFIX defaults to $N_PREFIX

[UiP9AV6Y]

i agree with the point about the lack of extendability in the future. i have implemented the changes based on your
suggestion.

[shadowspawn]

Looks good. I'll read again carefully and do some local testing before merging.