Pbs backend + bug fix

The bug-fix is a one-line change.

The PBS backend is a big commit which includes tests and all.
The test workflow(s) are a bit fragile: some failure trigger the clean-up too soon. But it is a testing problem; they should not block the PBS backend.

EDIT: grammar

`chain [a;b;c]` is like `a && b && c`.

That's documented there: http://seb.mondet.org/software/ketrew/api/Ketrew_edsl.Program.html