Nix flake updates

[ErrorNoInternet]

• Updates the cargo hashes again
• Updated Rust because matrix-sdk-common 0.7 was refusing to build on 1.67
• Used the current git commit hash as the version (appends -dirty if working tree is dirty)
• Small cleanups here and there

[benjajaja]

Thank you! I tested it 🚀

Maybe we should add an action to build the flake some time.

[ulyssa]

A GitHub action or some other way to test out building the Nix flake would be good! I've been thinking about trying to set up something on builds.sr.ht that runs after all of the GitHub actions have passed for testing out building on NetBSD and OpenBSD. If I get around to that, they also have NixOS images, so that could also take care of validating that the flake.nix works.