[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