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.