Automatically `make generate` when sigs.yaml is updated

### Describe the issue

(Filing issue after being prompted during Leads meeting)

After updating `sigs.yaml` it's necessary to run `make generate` which takes a really long time and a lot of disk space if you don't do it regularly.

It would be a lovely quality of life improvement for this to be done automatically, so that we could just commit `sigs.yaml` changes.