fixup! grammar in help
promote: correctly support default npm cache path

The default registry entry does not contain "cache" option, npm falls
back to the default path ($HOME/.npm). This commit modifies the
algorithm used to find the tarball downloaded by `npm cache add`
to support the default cache location too.

Fix lib/exec-npm to forward HOME and USERPROFILE environment variables.
Without that, npm uses /tmp/npm-{uid}/.npm as the default cache dir.