Fix regression in __pillar__ dunder

In salt.minion.Minion.__init__(), self._load_modules() is invoked, which
loads the execution modules via salt.loader.Loader.gen_modules().
However, this function also assigns the __pillar__ dunder to each
execution module, and the necessary pillar data wasn't being compiled
until the line after salt.minion.Minion._load_modules() was invoked.

This looks to have been changed in 4be8ff95, in order to be able to pass
the minion's functions to salt.pillar.get_pillar(), avoiding another
invocation of the loader in salt.pillar.Pillar.__init__(), but this came
at the expense of breaking the __pillar__ dunder.

Fixes #18867.
Merge pull request #19163 from terminalmage/issue18867

Fix regression in __pillar__ dunder