Fixes #5591.

For local-shell-script runners, the runner tries to ensure the script_action is executable by running:

        if script_action:
            script_local_path_abs = self.entry_point
            args = "chmod +x %s ; %s" % (script_local_path_abs, args)
            sanitized_args = "chmod +x %s ; %s" % (
                script_local_path_abs,
                sanitized_args,
            )

On readonly filesystems, such as when StackStorm is deployed in HA on Kubernetes - this chmod +x command fails:

stderr: "chmod: changing permissions of '/opt/stackstorm/packs/ansible/actions/ansible.py': Read-only file system

(Note since this command isn't && with the actual action, that still runs fine - there's just an error logged in the action output)

This is fixed, by only running this block if the filesystem is read-writeable - by adding the following to the conditional check:

bool(os.statvfs(self.entry_point).f_flag & os.ST_RDONLY)

Which returns True if the filesystem that self.entry_point (which is the script that will attemped to be chmod +x'ed) is on is readonly, and False otherwise.

This fix maintains functionally where there this worked previously (read-writeable fs), while fixing the bug on readonly fs. As per my comment on #5591, I think we should just drop this block all together - but that could be a change in 3.9.0 - for now this fixes the bug without changing anything else. :)