Fix site package on MYPYPATH check

[hauntsaninja]

This was a regression in 0.971

Fixes #13214

[hauntsaninja]

No unit test, since it's a little bit of a pain, but tested manually with:

mkdir repro
cd repro

mkdir stubs
echo 'x = 5' > stubs/mod.py

echo 'from setuptools import setup; setup()' > setup.py
echo 'from mod import x' > proj.py

python3 -m venv env
source env/bin/activate

python3 -m pip install -e .
python3 -m pip install -e ~/dev/mypy

MYPYPATH=$(pwd)/stubs python -m mypy proj.py

cd ..
rm -rf repro

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[vfazio]

Any chance this will drive a minor version bump?

[hauntsaninja]

I don't have the power to make releases, but I mentioned it in #13089 (comment)