provision: Fix source command on Windows.

tools/provision

@@ -116,6 +116,9 @@ the Python version this command is executed with."""
                                     venv_dir,
                                     venv_exec_dir,
                                     'activate')
+    # We make the path look like a Unix path, because most Windows users
+    # are likely to be running in a bash shell.
+    activate_command = activate_command.replace(os.sep, '/')
     print('\nRun the following to enter into the virtualenv:\n')
     print(bold + '  source ' + activate_command + end_format + "\n")