Support custom npm registry for npm and other installation.

[iamchathu]

Currently volta install node works and it installs default npm version. But if I try to install custom npm version it breaks as copertate proxy has blocked npm registry and we have our own npm registry with mirroring support. Installing local dependancies works as we have setup the npm config with custom registry. But volta seems a global config support to setup the custom npm registry.

[iamchathu]

Maybe we can use https://github.com/volta-cli/volta/pull/469 this to extend the support to package managers too?

[xiaoxiyao]

I also encountered this problem