Add komet.rust-stellar derivation that provides cargo and stellar in PATH

[juliankuners]

The nix flake provides the komet derivation that requires cargo and stellar to be available at runtime. Therefore, a user that installs komet must also install cargo with, e.g., rustup and install stellar with, e.g., cargo.

Akin to kontrol, an additional derivation can be provided in the nix flake that adds these two dependencies in the PATH of the executable to make the tools available at runtime. This has the benefit of increased reproducibility for use in, e.g., CI.

This pull requests introduces this derivation in the nix flake as the attribute komet.rust-stellar. It uses a rust distribution from github:oxalica/rust-overlay. Additionally, the rust target was changed from wasm32-unknown-unknown to wasm32v1-none due to respective upstream changes in stellar-cli introduced with stellar/stellar-cli#2022. Finally, the stellar executable that previously was provided in a nix develop shell, is instead now built from the new nix flake available in the stellar-cli repository.

The komet derivation that uses user-provided cargo and stellar can be build with nix build .#komet and the komet derivation that purely provides cargo and stellar can be built with nix build .#komet.rust-stellar.

[juliankuners]

This pull request also overrides stellar-cli due to a build issue. I created a respective pull request to fix it upstream as well: stellar/stellar-cli#2037.

[juliankuners]

@tothtamas28 Would you mind reviewing the nix changes?

[juliankuners]

Force pushed a rebase with the master branch.

[juliankuners]

Renamed soroban to stellar due to the change introduced recently in #83.