[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