Fix test.support.PythonSymlink on Windows source builds

Signed-off-by: Filipe Laíns <lains@riseup.net>

Author: FFY00

Lib/test/support/__init__.py

@@ -1716,9 +1716,10 @@ def _platform_specific(self):
                 ))
 
             self._env = {k.upper(): os.getenv(k) for k in os.environ}
-            self._env["PYTHONHOME"] = os.path.dirname(self.real)
+            home = os.path.dirname(self.real)
             if sysconfig.is_python_build():
-                self._env["PYTHONPATH"] = STDLIB_DIR
+                home = os.path.join(home, sysconfig.get_config_var('VPATH'))
+            self._env["PYTHONHOME"] = home
     else:
         def _platform_specific(self):
             pass