Create a GitHub app for nixpkgs-vet releases

[infinisil]

Also asked on Matrix:

I need a GitHub App to create releases for nixpkgs-vet (formerly nixpkgs-check-by-name), see here for more context.

Instructions:

• Create new App (direct link) with permissions Repository->Content->Read/Write, name can be "Nixpkgs-vet releases"
• Install the App on the org, give it access to only https://github.com/nixos/nixpkgs-vet
• Set the App ID as a new repository variable (direct link) named RELEASES_APP_ID
• Generate a private key for the App and set it as a new repository secret (direct link) named RELEASES_PRIVATE_KEY

I will then be able to use this GitHub Action to get ephemeral GitHub App installation tokens for CI