[Nix] Update nix flake lock, fix arcanist
Summary:
Fixes arcanist, avoid using php8.1 as it's current unsupported
Depends on D4181
Test Plan: nix develop && arc diff
Reviewers: varun, ashoat
Reviewed By: varun, ashoat
Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh, varun
Differential Revision: https://phab.comm.dev/D4292