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.

CC: @thatch45 @cachedout, please double-check this pull request, and let me
know if there is a better fix for this.