chore(3p): bump unstable channel to 2021-07-03

Didn't bother to update the stable channel as it is unused currently.

Changes required:

* tazjin/frog, grfn/modules/obs:
  Remove obs-v4l2sink as it has been integrated into upstream OBS and
  the package removed from nixpkgs subsequently (at least according to
  the `builtins.throw`-message).

Change-Id: I4335ed060eef2c4ff8ac55a68d894bcc3d8ef4a3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3243
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/nixos/frog/default.nix

@@ -248,7 +248,6 @@ in lib.fix(self: {
       nix-prefetch-github
       notmuch
       obs-studio
-      obs-v4l2sink
       openssh
       openssl
       pass