Setting `update_existing: true` in the `create-an-issue` GitHub Action
will avoid opening duplicate issues if an open issue already exists with
an identical title.
If no open issues match the title, then a new issue will be created.
This helps avoid spamming our issue tracker should there be a failure
when testing against Twisted's trunk.
This PR also pins the SHA of the `create-an-issue` action to mitigate
the risk of a malicious actor gaining access to JasonEtco's account.
See GitHub's page on security hardening third party actions for more:
https://docs.github.com/en/actions/learn-github-actions/security-hardening-for-github-actions#using-third-party-actions
Signed-off-by: Dan Callahan <danc@element.io>
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