Fix: Enable automatic skill selection if grounding fails

[matthias-mayr]

Previously the implementation relied on the "label" property. However, this property is set to the concrete instance, even if the compound skill did not specify an instance.

To explain this a bit more:
I have a use-case in which I have a skill that runs in a loop for different work pieces. In one part of the procedure, a skill needs to be automatically chosen depending on preconditions on the material properties. So it is used without specifying an implementation:

self.skill("MyFlexibleSkill", ""),

In the first iteration, it instantiates the correct implementation, but at a later point we might encounter a part that needs the other implementation & the grounding fails here:
https://github.com/RVMI/skiros2/blob/04353faa3f405ab970eb49915ce114354f9f5f94/skiros2_skill/src/skiros2_skill/core/skill_utils.py#L265-L272

This is fine, but when entering tryOther, a check is performed whether the label is set to be a specific skill:

https://github.com/RVMI/skiros2/blob/04353faa3f405ab970eb49915ce114354f9f5f94/skiros2_skill/src/skiros2_skill/core/skill_utils.py#L235-L240

Unfortunately this label is set to the currently set instance:

https://github.com/RVMI/skiros2/blob/04353faa3f405ab970eb49915ce114354f9f5f94/skiros2_skill/src/skiros2_skill/core/skill.py#L424-L430

Besides the entertaining comment, the _label property of the skill is still the original empty string and is not affected by the instance that was set before
So this PR proposes to use this instead.

[frvd]

I would rather fix it to check cut the link to an instance (has_instance) as a first thing in the tryOther function

[matthias-mayr]

That was also my first attempt, but it lead to some error. I can check what at it and look a bit more at it.