chore: prevent duplicate preview release comments

[matifali]

Summary

• Only create a PR comment if one with "✅ Preview binaries are ready!" doesn't already exist
• Prevents spam when the workflow runs multiple times

Test plan

• Trigger the workflow multiple times on a test PR
• Verify only one comment is created

[github-actions]

✅ Preview binaries are ready!

To test with modules: agentapi_version = "agentapi_119" or download from: https://github.com/coder/agentapi/releases/tag/agentapi_119

[github-actions]

✅ Preview binaries are ready!

To test with modules: agentapi_version = "agentapi_119" or download from: https://github.com/coder/agentapi/releases/tag/agentapi_119

[35C4n0r]

@matifali rather than checking all the comments, a better approach would be to get a variable from the previous step like this depending on the if condition, and then use it in the next step

echo "comment=true/false" >> "${GITHUB_OUTPUT}"

[github-actions]

✅ Preview binaries are ready!

To test with modules: agentapi_version = "agentapi_119" or download from: https://github.com/coder/agentapi/releases/tag/agentapi_119

[github-actions]

✅ Preview binaries are ready!

To test with modules: agentapi_version = "agentapi_119" or download from: https://github.com/coder/agentapi/releases/tag/agentapi_119

[matifali]

@35C4n0r as the workflow gets triggered by another workflow its not possible to test it in this PR. PRs can only run an updated workflow file if the trigger is on: pull_request

[35C4n0r]

LGTM