--python-executable doesn't discover plugins

[flexagoon]

Bug Report

When using --python-executable with globally installed mypy, it discovers the packages installed in the virtualenv, but not the installed plugins

To Reproduce

1. Use globally installed mypy
2. python -m venv .venv
3. .venv/bin/pip install pydantic
4. echo pass > main.py
5. In pyproject.toml:

[tool.mypy]
plugins = ["pydantic.mypy"]

6. mypy --python-executable .venv/bin/python main.py

Expected Behavior

It should pass with no errors, just like it does when installed inside of the virtualenv

Actual Behavior

pyproject.toml:1: error: Error importing plugin "pydantic.mypy": No module named 'pydantic'  [misc]
Found 1 error in 1 file (errors prevented further checking)

Your Environment

• Mypy version used: 2.1.0
• Python version used: 3.14.6