fix: ensure that targetjob is always forced. This fixes a bug causing…

… run-directive rules to not being executed even when enforced via e.g. -R. (#2448)

### Description

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### QC
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* [ ] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [ ] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).

snakemake/dag.py

@@ -177,6 +177,7 @@ def init(self, progress=False):
                 create_inventory=True,
             )
             self.targetjobs.add(job)
+            self.forcefiles.update(job.output)
 
         self.cleanup()