gh-96861: Check for unset sys.executable during venv creation. (GH-96887)

Lib/venv/__init__.py

@@ -128,6 +128,11 @@ class EnvBuilder:
         context.prompt = '(%s) ' % prompt
         create_if_needed(env_dir)
         executable = sys._base_executable
+        if not executable:  # see gh-96861
+            raise ValueError('Unable to determine path to the running '
+                             'Python interpreter. Provide an explicit path or '
+                             'check that your PATH environment variable is '
+                             'correctly set.')
         dirname, exename = os.path.split(os.path.abspath(executable))
         context.executable = executable
         context.python_dir = dirname