[dev env] add toolchain file

Summary: this file ensures all devs are using the same rustup toolchain. docs on toolchain file here: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file

Test Plan: confirmed that the specified rustup toolchain (1.75) overrode my previously installed toolchain inside the repo.

Reviewers: michal, bartek, ashoat

Reviewed By: michal, bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11004