re-assigning `worker_extra_args` instead of altering it with `+=`

[jolange]

Because worker_extra_args is assigned to a config value. With += the config value is changed every time, so the args will be appended again for each job resulting in #589.

It's a regression from #576.

Fixes #589

CC @guillaumeeb @theXYZT

[jolange]

@guillaumeeb , a bit of a silly request: Would you mind adding a label hacktoberfest-accepted to this PR to make it count for Hacktoberfest? (One can also tag a whole repo with hacktoberfest. That may attract contributions, but might also lead to unwanted "spam" PRs...)

[guillaumeeb]

I created a new label, is this OK?

[jolange]

@guillaumeeb perfect, thanks! :-)