Allow verify(-set) to work with alternate backends

[nwolverson]

Description of the change

Allow verify(-set) to work with alternate backends (when run in the context of a spago.dhall with backend set).

I've been using this change to verify the purerl package set

Checklist:

• Added the change to the "Unreleased" section of the changelog
• Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

P.S.: the above checks are not compulsory to get a change merged, so you may skip them. However, taking care of them will result in less work for the maintainers and will be much appreciated 😊

[f-f]

Looks great, thank you! 👏