[Nix] Fix substituters entry

Summary:
Most "documentation" you can find for nixConfig mentions use of
`extra-substituters`, however, this seems to be incorrect. See:
NixOS/nix#6672 (comment)

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

flake.nix

@@ -39,7 +39,7 @@
     # Add comm binary cache to avoid our pinned packages from needing
     # to be built by any consumer
     nixConfig = {
-      extra-substituters = [ "https://comm.cachix.org" ];
+      extra-trusted-substituters = [ "https://comm.cachix.org" ];
       extra-trusted-public-keys = [
         "comm.cachix.org-1:70RF31rkmCEhQ9HrXA2uXcpqQKGcUK3TxLJdgcUCaA4="
       ];