Add a www-deploy environment to the rustup repo

[Kobzol]

To enable deploying the rustup.rs website from GitHub Actions (https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Deploying.20rustup.2Ers.20from.20CI/with/579543904).

Also needs a corresponding simpleinfra PR.

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github
[INFO  rust_team::sync] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/rustup':
      Environments:
        ➕ Create: www-deploy
            Branches: www