De-couple atuin nix build from nixpkgs rustc version

[Sciencentistguy]

Closes #2089

This PR moves the nix flake to using a rust toolchain provided by fenix (https://github.com/nix-community/fenix), which downloads the toolchain declared in ./rust-toolchain.toml, removing the requirement on nixpkgs to update rustc for us to bump MSRV.

When the value in ./rust-toolchain.toml is changed, the nix build must be run locally once (with nix build -L). This will print the new sha256 hash for the new rust toolchain, that must then be inserted into flake.nix before the build will succeed. This may be automatable via a github action - that is not included in this PR.

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[ellie]

nice! thanks for sorting this out :D