Allow to change installation path

[qtc-de]

It seems that the installation process installs v to the current working directory. I think this choice is not optimal, as this could mess with module structuring or artifact collection. As an example, I was building documentation via v doc -m ., which works fine locally, but failed within the action, as v attempted to create docs for vlib modules too.

I would change the default installation path to a place that is usually not used by other pipeline commands. I have no experience with actions design, but a folder like /opt may be suitable. At least, it would be nice to be able to change the installation path using an option.

[JalonSolov]

With current V, the path has to be writable by the current user.