MeltanoHub vulnerable to command injection in the `test_dispatcher` GitHub Actions workflow

MeltanoHub is the source code for hub.meltano.com, the central place for Meltano plugins. Versions of the repo prior to commit 923820de8f64d753951fbbd54f7282a3d5f75173 were vulnerable to exfiltration of `GITHUB_TOKEN` with write permissions to the repository. The vulnerable workflow used pull_request_target, which runs in the context of the base repository with access to secrets. Commit 923820de8f64d753951fbbd54f7282a3d5f75173 fixes the issue. No known workarounds are available.