[Nix] Re-instate nixConfig

Summary:
Previously, the nixConfig did not work with just
the substitutor and public-keys defined. However, when looking
into how Cachix provisions a user environment, I noticed that
it defines the extra- variants as well. Having both the
non-prefixed and extra- prefix versions of the settings
seems to cover more corner cases.

Partial solution to https://linear.app/comm/issue/ENG-2623
May solve https://linear.app/comm/issue/ENG-2627

Test Plan:
Since we're testing whether a package will be download, we have to
remove the previous versions of those commands in the nix store. However,
removal of gc roots may be quite involved, one would need to:

• Exit all nix shells
• Remove any symlinks to the nix store (e.g. result -> /nix/store/...)
• Potential gc pinning from other tools (e.g. direnv)

# Remove gc roots
nix-collect-garbage
nix develop
# There shouldn't be any downloads

Reviewers: ashoat, atul, varun, max

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6286