Current behavior:
As mentioned in #74, our setup would really benefit from an adjustable runtime path. We have all our games in a common directory shared for all users where a user can just click a game and run it immediately (it's a gaming cave/computer class setup). Right now the runtime is automatically downloaded for every user separately to $HOME/.local/share/umu which means there are lots of duplicate runtimes in every user's home directory. It also means that every user after every update needs to wait for the runtime to download before their game starts.

Proposed behavior:
We would like to pre-download the runtime to a shared directory where the games are and point every game to use it from there, so that there is only one copy of it and it's always ready to use immediately for everyone. More generally we would like to avoid any use of user's home directory for downloads, as it's the admin's job to have everything ready. I noticed there is an internal env variable RUNTIMEPATH, which is always relative to $HOME/.local/share/umu. Maybe this could be expanded to allow paths anywhere in the filesystem, so that it would be analogous to PROTONPATH. If that's not possible, maybe another variable could be created for that purpose.

Workarounds:
Currently one could manually edit umu_consts.py to change UMU_LOCAL to point somewhere else. One could also symlink $HOME/.local/share/umu to a shared directory for every user.