Fix broken export-data admin command and add a test for it to CI 

Signed-off-by: Nick Barrett <nick@beeper.com>

We want to blacklist only known, consistent failures. We should deflake tests rather than ignoring them.

This creates a GHA workflow which runs at 8am every day, and runs mypy, trial and sytest against Twisted's current trunk. If any of the jobs fail, it opens an issue.

Should have been more systematic with my grepping.