When installing bin stubs, enable --env-shebang if File.dirname(Gem.ruby) is in $PATH

Problem

Generating absolute #! for rubygem's bin stubs makes it problematic for ruby switchers such as rbenv and chruby. Whenever a ruby is updated or perhaps moved/named, users must run gem pristin --only-executables to re-generating the gem bin stubs with the new path to the ruby executable.

Proposal

Too accommodate both users of ruby switchers and those who directly invoke ruby (ex: /path/to/ruby -S gem install ...), I propose the following behavior when generating bin stubs for newly installed gems:

1. Check if File.dirname(Gem.ruby) exists within ENV['PATH'].split(File::PATH_SEPARATOR).

   a. If so: enable --env-shebang behavior by default.
   b. If not: use "#!#{Gem.ruby}#{opts}" like normally.

Rationale

If the ruby's bin/ directory exists in $PATH it can be assumed this was intentional and that #!/usr/bin/env ruby would be equivalent to #!/usr/bin/ruby. If the ruby's bin/ directory is not in $PATH, then the user has likely executed it directly by calling /path/to/bin/ruby -S gem install ....

Start in lib/rubygems/installer.rb around the referenced bin-stub shebang logic. Trace how Gem.ruby and ENV['PATH'] are used when installing executables, then identify the existing installer coverage to update. Done means PATH detection selects env-style shebangs while direct Ruby paths retain absolute shebangs.