Let check-for-reproducer run only on newly created issues. (#38634)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38634

To reduce the noise of the `check-for-reproducer` bot, this restricts the event
that trigger the bot to only the issue creation.

Changelog:
[Internal] [Changed] - Let check-for-reproducer run only on newly created issues

Reviewed By: cipolleschi

Differential Revision: D47792374

fbshipit-source-id: 738a9342899e234022029e94f43b0dccebc21819