Add flake file

[jkarni]

Issue reference:
#93

Submission Checklist:

• Have you followed the guidelines in our Contributing document (for example, is your tree a clean merge)?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
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• Have you updated all of the cabal/nix infrastructure?
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[kvanbere]

I don’t know enough about nix to know whether this is okay or not, but one thing that stands out to me is the flake description - maybe it should be changed from the default.

Happy to merge it.

[kvanbere]

@ashercuedo

[ashercuedo]

Thanks for the pull request, I'll get onto it as soon as I can 👍

[jkarni]

one thing that stands out to me is the flake description - maybe it should be changed from the default.

Good call - changed to something more specific.