[Nix] Add shellcheck to dev shell

Summary: Allow for devs to run shellcheck from nix dev shell

Test Plan:

# from any branch
nix develop 'github:commE2E/comm?ref=jonringer/add-shellcheck'
shellcheck --help
which shellcheck # should point to nix store

Reviewers: abosh, atul, ashoat

Reviewed By: abosh, atul, ashoat

Subscribers: tomek, adrian

Differential Revision: https://phab.comm.dev/D4770