chore: rework virtual environments creation for tests (#2356)

As mentioned in https://github.com/pypa/cibuildwheel/pull/2347#discussion_r2029821701

Rework the way test venvs are created to reuse the same mechanisms as build venvs.

cibuildwheel/platforms/windows.py

@@ -265,21 +265,6 @@ def setup_python(
     env["PYTHON_ARCH"] = python_configuration.arch
     env["PIP_DISABLE_PIP_VERSION_CHECK"] = "1"
 
-    # upgrade pip to the version matching our constraints
-    # if necessary, reinstall it to ensure that it's available on PATH as 'pip.exe'
-    if not use_uv:
-        call(
-            "python",
-            "-m",
-            "pip",
-            "install",
-            "--upgrade",
-            "pip",
-            *constraint_flags(dependency_constraint),
-            env=env,
-            cwd=venv_path,
-        )
-
     # update env with results from CIBW_ENVIRONMENT
     env = environment.as_dictionary(prev_environment=env)
 
@@ -378,8 +363,7 @@ def build(options: Options, tmp_path: Path) -> None:
                 build_options.environment,
                 build_frontend.name,
             )
-            if not use_uv:
-                pip_version = get_pip_version(env)
+            pip_version = None if use_uv else get_pip_version(env)
 
             compatible_wheel = find_compatible_wheel(built_wheels, config.identifier)
             if compatible_wheel:
@@ -407,7 +391,7 @@ def build(options: Options, tmp_path: Path) -> None:
                 )
 
                 build_env = env.copy()
-                if not use_uv:
+                if pip_version is not None:
                     build_env["VIRTUALENV_PIP"] = pip_version
 
                 if constraints_path:
@@ -486,33 +470,16 @@ def build(options: Options, tmp_path: Path) -> None:
                 log.step("Testing wheel...")
                 # set up a virtual environment to install and test from, to make sure
                 # there are no dependencies that were pulled in at build time.
-                if not use_uv:
-                    call(
-                        "pip", "install", "virtualenv", *constraint_flags(constraints_path), env=env
-                    )
-
                 venv_dir = identifier_tmp_dir / "venv-test"
-
-                if use_uv:
-                    call("uv", "venv", venv_dir, f"--python={base_python}", env=env)
-                else:
-                    # Use pip version from the initial env to ensure determinism
-                    venv_args = [
-                        "--no-periodic-update",
-                        f"--pip={pip_version}",
-                        "--no-setuptools",
-                        "--no-wheel",
-                    ]
-                    call("python", "-m", "virtualenv", *venv_args, venv_dir, env=env)
-
-                virtualenv_env = env.copy()
-                virtualenv_env["PATH"] = os.pathsep.join(
-                    [
-                        str(venv_dir / "Scripts"),
-                        virtualenv_env["PATH"],
-                    ]
+                virtualenv_env = virtualenv(
+                    config.version,
+                    base_python,
+                    venv_dir,
+                    None,
+                    use_uv=use_uv,
+                    env=env,
+                    pip_version=pip_version,
                 )
-                virtualenv_env["VIRTUAL_ENV"] = str(venv_dir)
 
                 # check that we are using the Python from the virtual environment
                 call("where", "python", env=virtualenv_env)