Add triggers to .github/workflows/ (#1550)

This PR standardizes GitHub Actions triggers in workflow files within
`.github/workflows/`.

The goal is to ensure workflows run consistently on `push`,
`pull_request`, and `workflow_dispatch` events where appropriate.

This is part of a batch of pull requests across repositories owned by
the `android` organization on GitHub.

**Project Owner:** Please review the changes carefully to ensure they
are correct and appropriate for this project before approving and
merging.

* If you do not think this change is appropriate (e.g., a workflow
should NOT run on one of these triggers), please leave a comment
explaining why.
* If you think the goal is appropriate but notice a mistake in the
implementation, please leave a comment detailing the mistake.

Author: bentrengrove

.github/workflows/Release.yml

@@ -1,6 +1,7 @@
 name: GitHub Release with APKs
 
 on:
+  workflow_dispatch:
   push:
     tags:
     - 'v*'

.github/workflows/copy-branch.yml

@@ -5,6 +5,7 @@ name: Duplicates main to old master branch
 # Controls when the action will run. Triggers the workflow on push or pull request
 # events but only for the main branch
 on:
+  workflow_dispatch:
   push:
     branches: [ main ]