optimize `encrust fatten` by downloading `universal2` binaries preferentially

[glyph]

There's a strategy described in this comment:

https://github.com/pypa/pip/issues/5453#issuecomment-833152775

which boils down to

python3 -m pip download --only-binary=:all: --platform=macosx_10_9_universal2 --platform=macosx_11_0_universal2 --no-cache-dir $STUFF --dest ./.wheels/downloaded/universal2
python3 -m pip install --only-binary=:all: --no-cache-dir --no-index --find-links ./.wheels/downloaded/universal2 $STUFF

I believe that this would directly download official binaries in more cases where all 3 architectures are provided, rather than fusing them.