adding an envfile or similar to change default behavior

[tcsenpai]

I think it would be nice to have an .env file or similar (or something to pass to umu-run by the way) that allows us to choose for example the default sniper download path and other choices. This would be useful for launchers and 3rd party tools too, providing a degree of customization that may be useful.

Right now umu_consts.py contains many of the default options as constants (indeed). With a more flexible approach, launchers can ship their runtimes too or have multiple runtimes if they need to.

[R1kaB3rN]

One of the concerns of changing the default paths of the runtime is that it would result in runtimes being in multiple places which are ~700MB in size (uncompressed). To reduce the amount of data stored in the system, I think it would require centralizing the location of runtimes and deduplicating the content in some way (e.g., libostree from Flatpak). To support it, _v2-entry-point and the run-in* files would probably need to be reimplemented as well to control the storage of the variable directory and other things.

In the end, it would sorta look like Flatpak's functionality and I'd rather not reimplement what Flatpak already does especially if such a feature won't really be useful (yet). Therefore, I think this feature is more suited to be Flatpak-specific and should be explored once we start building the umu runtime.