Fix typo "c" -> "user_config" for --extra_runtime_dependencies

[olleolleolle]

Description

The CLI option parsing code for --extra-runtime-dependencies had a typo in it.

There was a c here, and it seemed, out of place.

o.on "--extra-runtime-dependencies GEM1,GEM2", "Defines any extra needed gems when using --prune-bundler" do |arg|
  c.extra_runtime_dependencies arg.split(',')
end

Comparing with --prune_bundler, which was mentioned next to this long option, we could try using user_config instead.

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[olleolleolle]

@nateberkopec I inspected this, and just was unable to locate c.

I compared with user_config.prune_bundler and picked user_config for this.

[composerinteralia]

This looks similar to #2039. We got rid of c in cda9317, but the PR to add this option (#1105) was opened before that change was made and the branch was probably never updated.

I will search for any more lingering references to c that are broken.

[nateberkopec]

Needs a test 👍

[olleolleolle]

(I added the simplest test for this code path - something that would have failed with the old code.)

[nateberkopec]

Thanks @olleolleolle 🎉