feat: allow keys from env

[mikn]

Issue number:

Closes #464

Description of changes: This adds support for getting the key for signing repositories from the env as well as file/kms etc.

Testing done: Has been used in a production setting for a few months.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[cbgbt]

Seems like the CI is unhappy, you just need to run cargo fmt (and possibly also cargo clippy if you haven't yet).

[jmt-lab]

Looks like a simple forgotten import, thanks for contributing i'll keep an eye to give second approval once you update

[mikn]

Sorry I am wasting so many compute minutes on your github worker :D I am not great at running the upstream tooling before submitting commits. I will be better, mark my words.

[jmt-lab]

Merged! Thank you so much for your contribution!