Choose a single Rust nightly instead of stable/nightly mix

[maxammann]

Integrating Nix shows problems because the project depends on a nightly and stable verison of Rust right now.

The nightly verison is needed because of:

• web uses the build-std feature for multithreading
• we use the nightly rustfmt
• android uses nightly for unknown reasons

Until maplibre-rs is even remotely stable it makes sense to use onyl the nightly I beleive and simplify setup.

In nix we can get the path to Rust like that:

 export RUST_PATH="${rust}/bin";
 export RUST_SRC_PATH="${rust}/lib/rustlib/src/rust/library";