[Nix] Fix substituters entry

Summary:
Most "documentation" you can find for nixConfig mentions use of
extra-substituters, however, this seems to be incorrect. See:
https://github.com/NixOS/nix/issues/6672#issuecomment-1251573660

This also removes the

warning: ignoring untrusted substituter 'https://comm.cachix.org'

output that you might see when doing any nix command.

Test Plan:

nix develop
# may get prompted about nixConfig values
# assert "ignoring untrusted substituter 'https://comm.cachix.org'" doesn't appear

Reviewers: atul, varun, abosh, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5183