Avoid duplicate issues from Twisted trunk failures (#10672)

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>

.github/workflows/twisted_trunk.yml

@@ -82,8 +82,9 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
-      - uses: JasonEtco/create-an-issue@v2
+      - uses: JasonEtco/create-an-issue@5d9504915f79f9cc6d791934b8ef34f2353dd74d # v2.5.0, 2020-12-06
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         with:
+          update_existing: true
           filename: .ci/twisted_trunk_build_failed_issue_template.md

changelog.d/10672.misc

@@ -0,0 +1 @@
+Run a nightly CI build against Twisted trunk.