ci: Use nixpkgs-unstable instead of nixos-unstable

Signed-off-by: Naomi Roberts <mia@naomieow.xyz>

Changed files
+1 -1
.tangled
workflows
+1 -1
.tangled/workflows/deploy.yaml
··· 5 5 engine: "nixery" 6 6 7 7 dependencies: 8 - github:NixOS/nixpkgs/nixos-unstable: 8 + github:NixOS/nixpkgs/nixpkgs-unstable: 9 9 - gleam 10 10 - erlang_28 11 11 - beam28Packages.rebar3