pybind/mgr/dashboard/frontend: add NPM_CACHEDIR envvar, use in bwc

pybind/mgr/dashboard/frontend: add NPM_CACHEDIR envvar, use in bwc

Add an optional NPM_CACHEDIR environment variable to serve as the
cache parameter for npm in the dashboard frontend build.  The idea
is to allow it to persist across builds so that we decrease the load
on registry.npmjs.org, which has been throttling our requests when
using build-with-container.py, and also hopefully improve the time
of the frontend npm operations.

build-with-container.py also grows a --npm-cache-path option to allow
setting it for container builds and passing the envvar to the build.

Fixes: https://tracker.ceph.com/issues/72298
Signed-off-by: Dan Mick <dan.mick@redhat.com>