fix: never call pip directly (#2537)

You can't change the running process in Windows. In pip 25.2, the format of the generated file changes, so this now breaks if you try to update to the latest pip (which can happen if deps are unpinned).

Author: henryiii

cibuildwheel/venv.py

@@ -143,6 +143,8 @@ def virtualenv(
     venv_env["VIRTUAL_ENV"] = str(venv_path)
     if not use_uv and pip_version == "embed":
         call(
+            "python",
+            "-m",
             "pip",
             "install",
             "--upgrade",