Allow running publish-unstable on workflow_dispatch

[NullVoxPopuli]

Sometimes the GitHub API fails with Error: 429 Too many Requests. This can happen during the publish-unstable workflow, which isn't great, but we can wait for quota to recover a bit, and re-run the workflow manually.

This is done with the workflow_dispatch trigger, as implemented in this PR.

I've also done this on on my own repo, for syncing a branch that tests the latest unstable release of all the embroider packages.

I first learned about this strategy from: https://dev.to/this-is-learning/manually-trigger-a-github-action-with-workflowdispatch-3mga

Here are the docs on workflow_dispatch
And How to manually run a workflow

[mansona]

I don't know if we need this change 🤔 we discussed this in the meeting and #1393 is the actual fix that we need