Authentication with Cachix doesn’t require GitHub

[toastal]

They support email as well.

Closes

other changes

• [ ] Every newly introduced function needs to have a documentation
• [ ] The impure tests via nix run .#offchain:test were run and all are passing.
• [ ] Where applicable, grammar, punctuation, and spelling within code and comments should be plausibly correct.
• [ ] The issue(s) that the work is addressing should be linked in the PR description.
• [ ] Any significant changes beyond what's clear from the linked issue(s) are documented in the PR description