Add a small fix for virtual envs pointing at a base Python install

poetry/utils/env.py

@@ -723,6 +723,19 @@ def _bin(self, bin):  # type: (str) -> str
         """
         bin_path = (self._bin_dir / bin).with_suffix(".exe" if self._is_windows else "")
         if not bin_path.exists():
+            # On Windows, some executables can be in the base path
+            # This is especially true when installing Python with
+            # the official installer, where python.exe will be at
+            # the root of the env path.
+            # This is an edge case and should not be encountered
+            # in normal uses but this happens in the sonnet script
+            # that creates a fake virtual environment pointing to
+            # a base Python install.
+            if self._is_windows:
+                bin_path = (self._path / bin).with_suffix(".exe")
+                if bin_path.exists():
+                    return str(bin_path)
+
             return bin
 
         return str(bin_path)