make cache dir configurable

[UiP9AV6Y]

currently CACHE_DIR is hardcoded to
use $N_PREFIX/n/versions as persistence
layer for node versions.

we use n in a CI context and manage
the node versions with a privileged user
while the CI user is only allowed to
switch between existing/cached versions.

currently we solved this with $N_PREFIX
being owned by the CI user and
$N_PREFIX/n being owned by the
privileged user to prevent
writing/downloading new versions during
CI jobs.

it should probably be sufficient to
make CACHE_DIR configurable just like
N_PREFIX, defaulting to the currently
hardcoded value (and also rename it to
N_CACHE_DIR to be unambiguous)

p.s.: yes i know, you can bypass this
restriction by setting N_PREFIX to a
different location, but that is a
different problem.

[shadowspawn]

It feels an indirect partial solution to your stated goals (as you say!), but separating the cache location and the install location seems fairly reasonable for flexibility in configuration. They are different concerns.

[shadowspawn]

Released in v8.1.0. Thanks for your contributions. 😄