`base: main` made every push to main rescan the whole history, where Nova's
own Infura keys are still live, so Secret Scan was red on every push and a
new leak would have read as the same old red.
- push and pull_request: no `base`, so the action scans only the commits the
event brings
- schedule: scan a `git archive HEAD` copy of today's tree
- pin the action to v3.99.2 by commit and the image to 3.99.2 (was @main and
latest)