GitHub-CI/build_nix_NixOS-stable: rename job to build_nix_NixOS-unstable

Turns out the nixos/nix Docker image is using
https://nixos.org/channels/nixpkgs-unstable as the default channel
instead of stable. So this job was always testing builds on/against Nix
unstable which is now correctly reflected in the job name.

.github/workflows/auto-ci-builds.yml

@@ -331,7 +331,7 @@ jobs:
       run: |
         dpkg-buildpackage -us -uc -rfakeroot --build=binary
 
-  build_nix_NixOS-stable:
+  build_nix_NixOS-unstable:
     runs-on: ubuntu-latest
     container: nixos/nix
     steps: