[CI] Auto-accept flake config

Summary:
Nix by default only polls for user input when confirming the values,
in a CI environment there's no way to progress the builds without passing
the '--accept-flake-config' value explicitly.

See: https://github.com/NixOS/nix/issues/7071

Test Plan: https://buildkite.com/comm/tunnelbroker-unit-tests/builds/50#01835b9d-1109-4c97-9c3a-444f70fb6a82 passes

Reviewers: atul, ashoat, abosh, varun

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D5194