[RFC] Run Coverity only twice per week.

Follow-up from neovim/neovim#1157: Coverity will run on Wednesdays and Saturdays only.
I like the way we wrapped the dependencies in an explicit `is_ci_build` block so it's obvious to the reader what's going on. Can we do that here too?

```
if_date_ok {
    ...
}
```