This gets in the middle if we ever start allowing to build as if using a
different RubyGems version than the one being run.

This could be useful to make `gem rebuild` a little more usable, and
it's already done by Bundler specs which already make this method a noop
when they need this.

I'm not sure forcefully setting this, even if user explicitly specified
something else is helpful.

Since this could potentially prevent gems explicitly setting a constant
RubyGems version from building, I changed the error of incorrect
RubyGems version from a hard error to a warning, since it will start
happening in those cases if we stop overwriting the version.

https://github.com/rubygems/rubygems/commit/45676af80d