Describe the bug

Puma should always prefer the rackup file specified by the CLI, but if the rackup DSL command is used, it does not.

Related: #1401

Puma config:

Using the test fixtures in the repo:

rackup "test/rackup/hello-env.ru"

To Reproduce

bin/puma -C my_rackup_config.rb test/rackup/hello.ru

...should return a Hello World! response, but runs the hello-env app instead.