Allow custom installation location

[shpoont]

Since pkgx install is simply checking if the user has permissions to write to /usr/local it has a very confusing behavior. When there are more than one users in the system it will have conflicting results because some packages can be already installed by another user to the same folder.

It would be useful to have a simple PKGX_INSTALLATION_DIR showing it where to install/uninstall packages.

[felipecrs]

I would say even pip, npm, cargo and whatnot should also be patched to respect this environment variable (when installed by pkgx).