Use .rultor.yml from a PR instead of getting it from a default branch

[volodya-lombrozo]

No so long time ago we faced with the problem of updating .rultor.yml. The problem is that we can not update .rultor.yml by using default development approach - through PR`s:

this particular commit rultor wouldn't catch, because it uses .rultor.yml from master. I don't push to master, but this one was the only possible option

objectionary/eo#1949 (comment)

This is quite risky: even rare direct pushes to a default branch can break all CI pipelines and break development process.

---

I'm aware about that problem, but since it doesn't explain possible threats, I don't see vulnerabilities in that approach.

[volodya-lombrozo]

@yegor256 Could you have a look, please? What do you think?

[yegor256]

@volodya-lombrozo take a look: #1039