[install.sh] Sanity check that `/usr/local/etc/` can be written too

Very odd issue that currently debugging for a user, they ran install script but the package installed but no scenarios, parsers or machines were configured.

They couldnt even `cd /usr/local/etc/crowdsec` upon reboot they can now access it, so maybe we need to add a check that `/usr/local/etc` is writable? 🤷🏻 clutching at straws here 😅 