Allow author name and email to be set via environment variables

[ChrisPearce]

It would be helpful for the commit author and email to be able to be set using environment variables.

Currently these are hard-coded in the /generic-update-script.rb file with the line: author_details: { name: "Dependabot", email: "[email protected]" }

[JaredAAT]

I suspect at this point, we need to start looking at using the dependabot.yml file to set variables rather than having so many environment variables floating around.