Always use Gem::Installer mutex

If Gem::Installer is subclassed (as in Bundler) then @install_lock will
not be initialized and Bundler will not be able to install gems.

Using an explicit class allows subclassing to work transparently.

See
https://github.com/rubygems/rubygems/commit/d96e0d5f48678fa0eefc4862378974294fc0a2cf#commitcomment-7729421