chore(toolchain): bump rust toolchain + simple Nix

[bhoudebert]

Hello,

Would you mind a little rustup bump?

I ran rustfmt with tab_spaces=4 (seems close to your preferences, even if I strongly advise using only 2).

Tried to update our CI part with blind search/replace rust toolchain version. Still I added a little convenient part to allow discarding build if a subsequent commit is done on the same branch and the previous build is still running.

I also added some Nix files just for providing in the shell some useful files (without having to manually install them on the host). These files are totally optional.

Thanks for your work, up to you to do whatever you want from this.