Update minimum rust version and build process

[aljazerzen]

This PR:

• sets rust-version = "1.76" for all published crates in this repo,
• adds flake.nix (and flake.lock) that provide dev shell,
• 3 available shells: default (from rust-toolchain.toml), minimal (that matches rust-version) and beta,
• justfile that defines all tests that previously ran in GH actions,
• update GH actions to use these shells and the justfile.

Now it is possible to run all tests that run in GH actions to also run locally.