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
e920ee3289