It would be simpler to do `options[:lockfile] ||= ENV["BUNDLE_LOCKFILE"]`,
but that doesn't work as `options` is frozen.

Fixes https://github.com/ruby/rubygems/pull/9117

https://github.com/ruby/rubygems/commit/6e3603a0e9

This allows for specifying the lockfile to read and write. It mirrors
the --gemfile option, and has higher priority than the lockfile
method in the Gemfile. It also mirrors the bundle lock --lockfile
option.

When the --lockfile option is used, it is applied twice. First, before
the Gemfile is read, to specify the lockfile to operate on, and again
after the Gemfile is read, so that if the Gemfile has a lockfile
method that overrides the defintion's lockfile, the --lockfile option
still has higher precedence.

https://github.com/ruby/rubygems/commit/17acdd4a89

This specifies the lockfile location. This allows for easy support
of different lockfiles per Ruby version or platform.

https://github.com/ruby/rubygems/commit/b54d65bc0a

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Colby Swandale <996377+colby-swandale@users.noreply.github.com>

This allows you to specify the lockfile to use. This is useful if
you want to use different lockfiles for different ruby versions or
platforms. You can also skip writing the lockfile by using a false
value.

https://github.com/ruby/rubygems/commit/2896aa3fc2

Co-authored-by: Colby Swandale <996377+colby-swandale@users.noreply.github.com>

https://github.com/rubygems/rubygems/commit/573ffad3ea

For synopsis, subcommand and flag documentation like other commands do.

https://github.com/rubygems/rubygems/commit/f528029756

There already different ways of toggling off this behavior, like setting
`bundle config clean false`, or configuring Bundler to install to system
gems with `bundle config path.system true`.

https://github.com/rubygems/rubygems/commit/6daa09f60a

https://github.com/rubygems/rubygems/commit/3e4687616a

https://github.com/rubygems/rubygems/commit/9ed20bddab