allow interactive debugging in the CLI

[jakecoffman]

When running in the CLI with script/dependabot, this allows us to add debugger in the code and then invoke bin/run fetch_files --debug to start a debugging session once that line is hit.

[mattt]

Agreed with what @jeffwidman said. I think we can wait for the Ruby 3.1 upgrade to use Ruby's built-in debug gem.

[jeffwidman]

#5447 is merged, so this should be unblocked.