Log backend command on build

[nwolverson]

Description of the change

Add logging of the backend used (when present) for the build command. I was expecting the -v output to show the backend command being run, similarly to the purs command

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, thanks! 💯