fix: make update.sh respect custom runtime directory

[ryanccn]

This PR makes update.sh work in custom runtime directories instead of writing to the default (~/invokeai).

update.bat has this fix but update.sh does not.

https://github.com/invoke-ai/InvokeAI/blob/0439b51a26e3c227588a22897309928519ad8412/installer/templates/update.bat.in#L42