After https://github.com/NixOS/nix/pull/10071, the CI was trying to push ghcr.io/nixos/nix:master for backwards-compatibility, but the image was not tagged as such, causing the job to fail. Fix this.
After https://github.com/NixOS/nix/pull/10071, the CI was trying to push ghcr.io/nixos/nix:master for backwards-compatibility, but the image was not tagged as such, causing the job to fail. Fix this.