[Nix] Add backwards compatible stub for older nix versions

Summary:
Nix 2.7 introduced .default being the default attribute for flake
outputs. For example, rather than having overlay the default
flake output is now overlays.default.

See: https://nixos.org/manual/nix/stable/release-notes/rl-2.7.html

This change is to add a "compatible stub" for the CI machines, which are running
an older version of nix.

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

Reviewers: atul, varun, max, abosh, ashoat, O1 Nix

Reviewed By: varun, ashoat, O1 Nix

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D5196