env: handle Debian's posix_local scheme

The issue and solution is the same as OSX.
The venv scheme wasn't standardised yet, so it's not using it yet.

src/build/env.py

@@ -307,6 +307,13 @@ def _find_executable_and_scripts(path: str) -> Tuple[str, str, str]:
         # See https://bugs.python.org/issue45413
         # and https://github.com/pypa/virtualenv/issues/2208
         paths = sysconfig.get_paths(scheme='venv', vars=config_vars)
+    elif 'posix_local' in scheme_names:
+        # The Python that ships on Debian/Ubuntu varies the default scheme to
+        # install to /usr/local
+        # But it does not (yet) set the "venv" scheme.
+        # If we're the Debian "posix_local" scheme is available, but "venv"
+        # is not, we use "posix_prefix" instead which is venv-compatible there.
+        paths = sysconfig.get_paths(scheme='posix_prefix', vars=config_vars)
     elif 'osx_framework_library' in scheme_names:
         # The Python that ships with the macOS developer tools varies the
         # default scheme depending on whether the ``sys.prefix`` is part of a framework.

tests/test_env.py

@@ -5,6 +5,7 @@
 import platform
 import subprocess
 import sys
+import sysconfig
 
 import pytest
 
@@ -56,6 +57,17 @@ def test_can_get_venv_paths_with_conflicting_default_scheme(mocker):
     assert get_scheme_names.call_count == 1
 
 
+@pytest.mark.skipif('posix_local' not in sysconfig.get_scheme_names(), reason='workaround for Debian/Ubuntu Python')
+def test_can_get_venv_paths_with_posix_local_default_scheme(mocker):
+    get_paths = mocker.spy(sysconfig, 'get_paths')
+    # We should never call this, but we patch it to ensure failure if we do
+    get_default_scheme = mocker.patch('sysconfig.get_default_scheme', return_value='posix_local')
+    with build.env.IsolatedEnvBuilder():
+        pass
+    get_paths.assert_called_once_with(scheme='posix_prefix', vars=mocker.ANY)
+    assert get_default_scheme.call_count == 0
+
+
 def test_executable_missing_post_creation(mocker):
     venv_create = mocker.patch('venv.EnvBuilder.create')
     with pytest.raises(RuntimeError, match='Virtual environment creation failed, executable .* missing'):