Add a workflow trigger to release branches (#853)

Summary:
So that release only or cherry pick changes like https://github.com/pytorch/executorch/pull/849 is run when pushing to `release/*` branch.

Commits from an arbitrary branch can be shown on HUD by changing the branch name, for example https://hud.pytorch.org/hud/pytorch/executorch/release%2F0.1

Pull Request resolved: https://github.com/pytorch/executorch/pull/853

Reviewed By: mergennachin

Differential Revision: D50203529

Pulled By: huydhn

fbshipit-source-id: 5b74581efef0d83dae81ec34fb1fac4b3e8c06cb