use the provided GITHUB_TOKEN instead of a provided one

[Aubron]

Recently updating these shopify provided github tokens to GITHUB_TOKEN, which is provided by github in the run context of the job, has been needed to get old jobs running: I think a credential changed or expired at the org scope.

Unsure if this will fix the problem, but should give us next steps.

Also added a workflow_dispatch to this action with a configurable branch name, so we can test the deploy flow from PRs in the future, no more CI golf.

[shopify-shipitnext]

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