main: Added nix shell script to make building the binaries easy on…

[corynthian]

… NixOS.

Motivation

The Makefile is currently specific to Ubuntu. This change adds a NixOS script which pulls in all the deps required to build the codebase.

Test Plan

Run nix-shell in the root on NixOS then run:

cargo build -p diem-node -p tower -p backup-cli -p ol -p txs -p onboard --release

If running for the first time run rustup update prior to the cargo build command.

Related PRs

None?

[corynthian]

Just tried it on a new host, for some reason had to run rustup install 1.52.1 from the rust-toolchain after rustup update.

[0o-de-lally]

Super helpful. If you don't need it in the root of the project, I would move it to ol/util where we have some .sh helpers.