Nix build and cachix push workflow

[Xpirix]

Description

This workflow will push the Nix QGIS build to cachix, so LTR and Latest will not build from source anymore. This will require:

1. Sign up to cachix and log in
2. Go to "Caches" tab and click on "New Cache"
3. Fill the form the following:

• Unique name: qgis
• Visibility: Public
• Signing: Managed by Cachix

4. Once cache is created, click on it and go to "Settings > Auth Tokens"
5. Create auth token with "Write+Read" permissions and no expiration
6. Copy the token to clipboard and create a secret in https://github.com/qgis/QGIS Github account (Settings > Secrets > Actions) called CACHIX_AUTH_TOKEN with the token as the value.
7. Copy the cache Public Key from Cachix and update flake.nix.
8. Create the PR in the upstream repo

Cc @timlinux

[github-actions]

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit dd6dc7e)

[Xpirix]

This shouldn't be needed because the nix flake check will do this step.

[Xpirix]

Nix flake check is building all outputs and packages is one of them.

[Xpirix]

Cachix has a post job that will push to cachix.