Show host python under core section as constant

Signed-off-by: Bernát Gábor <bgabor8@bloomberg.net>

docs/changelog/2630.bugfix.rst

@@ -0,0 +1,2 @@
+The core tox configuration now contains ``host_python`` key showing the host python executable path -
+by :user:`gaborbernat`.

src/tox/run.py

@@ -41,6 +41,7 @@ def main(args: Sequence[str]) -> int:
     result = provision(state)
     if result is not False:
         return result
+    state.conf.core.add_constant("host_python", "the host python executable path", sys.executable)
     handler = state._options.cmd_handlers[state.conf.options.command]
     result = handler(state)
     return result

tests/session/cmd/test_show_config.py

@@ -219,3 +219,10 @@ def test_show_config_timeout_custom(tox_project: ToxProjectCreator) -> None:
 def test_show_config_help(tox_project: ToxProjectCreator) -> None:
     outcome = tox_project({"tox.ini": ""}).run("c", "-h")
     outcome.assert_success()
+
+
+def test_show_config_core_host_python(tox_project: ToxProjectCreator) -> None:
+    project = tox_project({"tox.ini": ""})
+    outcome = project.run("c", "--core", "-e", "py", "-k", "host_python")
+    outcome.assert_success()
+    assert f"host_python = {sys.executable}" in outcome.out