[maintenance] E2E tests aren't running on Version Packages PR

We use `@changesets/action` to open a PR that manages our versioning, which gets created/updated during a step of https://github.com/cloudflare/wrangler-action/blob/main/.github/workflows/release.yml. We also run our E2E tests via a workflow that runs on `pull_request` events. It turns out that [GitHub avoids triggering workflows](https://docs.github.com/en/actions/using-workflows/triggering-a-workflow#triggering-a-workflow-from-a-workflow) from events generated by other workflows when using the automatically provided `GITHUB_TOKEN`, which means E2E tests don't get triggered to run any time the Version Packages PR is opened or updated. Sad!

However, [in late 2022 GitHub made an exception](https://github.blog/changelog/2022-09-08-github-actions-use-github_token-with-workflow_dispatch-and-repository_dispatch/) to allow workflows to be triggered via `workflow_dispatch` events even if `GITHUB_TOKEN` was used.

We should add a step to our `release.yml` workflow to send a `workflow_dispatch` event for the E2E workflow with a `ref` of `changeset-release/main` (following the example in the linked blog post above). We'll also need to ad a new `on: workflow_dispatch` trigger to our E2E workflow.