[nix] Lint line length

Summary:
Limit line length to 80 characters

https://phab.comm.dev/D4294#inline-27088

Depends on D4294

Test Plan: None, code hygiene

Reviewers: ashoat, varun

Reviewed By: ashoat, varun

Subscribers: palys-swm, Adrian, atul, yayabosh, varun, ashoat

Differential Revision: https://phab.comm.dev/D4424