ci default tag

[zimbatm]

• ci: fix docker default tag

Motivation

Fix #10042

Context

Docker uses "latest" as the default label instead of "master".

This change will allow to docker run ghcr.io/nixos/nix without having to specify the label.

This would also put it in line with the doc: https://github.com/zimbatm/nix/blob/9853330e016ccea5c58c20f2901c98d04f4f3fa8/doc/manual/src/installation/installing-docker.md?plain=1#L7-L8

Priorities and Process

Add :+1: to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

[zimbatm]

This would break the Docker Hub listed in https://github.com/zimbatm/nix/blob/9853330e016ccea5c58c20f2901c98d04f4f3fa8/doc/manual/src/release-notes/rl-2.6.md?plain=1#L20-L21

Potentially we could push to both tags on Docker Hub, it doesn't take much more storage.