[CI] Add default nix paths to Nix CI jobs

Summary:
Update tunnelbroker nix ci job to supply default PATH for nix.
Nix normally supplies these settings through sourcing /etc/zshrc or /etc/bashrc.
However, files under /etc are subject to destructive updates when macOS is updated.

Test Plan:
tunnelbroker: https://buildkite.com/comm/tunnelbroker-unit-tests/builds/353#0185325f-1bd3-4525-9c7d-7d04e38465d8
identity: https://buildkite.com/comm/identity-tests-nix/builds/115

Reviewers: varun, atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5962