RubyGems mutates RbConfig and sets it to incorrect values

[eregon]

RubyGems mutates RbConfig sitearchdir and sitelibdir when building a C extension gem.

RbConfig should never be mutated, as it can lead to various issues.
One of them is actually RubyGems no longer being able to find where to insert in $LOAD_PATH:

rubygems/lib/rubygems.rb

Lines 573 to 581 in c98464e

	def self.load_path_insert_index
	$LOAD_PATH.each_with_index do |path, i|
	return i if path.instance_variable_defined?(:@gem_prelude_index)
	end
	index = $LOAD_PATH.index RbConfig::CONFIG['sitelibdir']
	index || 0
	end

This issue was made more obvious as rake-compiler copied this logic:
rake-compiler/rake-compiler#202

Could these global mutations be avoided by RubyGems?
Maybe make install sitearchdir=... or so would work instead?

This might also come up as an issue as loading RubyGems is made more lazy (https://bugs.ruby-lang.org/issues/18568).

I will abide by the code of conduct.

[deivid-rodriguez]

I tried the approach suggested at rake-compiler/rake-compiler#202 and it seems to work and make things simpler, so I'm good with doing this.

[eregon]

Great!