update Nix build for Rust 2021 edition requirement

Switch to oxalica/rust-overlay as it's easier to select Rust versions. Dev shell includes src for standard libraries.

Created by  rohan  on December 2, 2021
MUYNSHG26H3WEDIQZKKD57H4XCMOVGLDDXZ4SZEFQ6MDUN3HSYEAC
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