add tuf-root-signing-staging to root-signing repo

[cpanato]

Summary

• add tuf-root-signing-staging to root-signing repo

follow up of https://github.com/sigstore/community/pull/348

cc @jku

[github-actions[bot]]

:tropical_drink: preview on sigstore-github-sync/sigstore/github-prod

Pulumi report

``` Previewing update (sigstore/github-prod) View Live: https://app.pulumi.com/sigstore/sigstore-github-sync/github-prod/previews/dd89c30d-2fe9-4541-8309-334780a3407f @ Previewing update....... pulumi:pulumi:Stack: (same) [urn=urn:pulumi:github-prod::sigstore-github-sync::pulumi:pulumi:Stack::sigstore-github-sync-github-prod] @ Previewing update.... + github:index/teamRepository:TeamRepository: (create) [urn=urn:pulumi:github-prod::sigstore-github-sync::github:index/teamRepository:TeamRepository::root-signing-tuf-root-signing-staging] permission: "maintain" repository: "root-signing" teamId : "8790813" Resources: + 1 to create 570 unchanged ```

[jku]

I've made #351 (with the teamid from this PR) : that is my best guess on what we need to setup a proper staging TUF repository.

I suggest this is closed and discussion happens in 351

[cpanato]

@jku i see, ok i will close this PR