fix: download runtime to private cache for flatpak

[R1kaB3rN]

Fixes #219

[zocker-160]

I think it would be better to use XDG_CACHE_HOME instead.
Inside Flatpak this resolves to the same path that you are hardcoding right now, which is /home/USER/.var/app/APPID/cache and use the default path when not set.

[R1kaB3rN]

The problem with $XDG_CACHE_HOME is that we could be downloading to a client's private Flatpak cache directory. For instance, Heroic and Lutris sources umu-launcher in its runtime and when they call umu-run, the value of $XDG_CACHE_HOME will be Heroic's instead of umu-launcher. This could cause the runtime to be downloaded there, which is not what we want. We want to centralize all Steam Runtime installations in a single place, so there isn't runtimes everywhere.

[zocker-160]

@R1kaB3rN but isn't that actually desirable?

When I use lutris Flatpak, the folder it tries to access .var/app/org.openwinecomponents.umu.umu-launcher does not exist.

Do the default permissions even allow to create and write to that folder?

We want to centralize all Steam Runtime installations in a single place, so there isn't runtimes everywhere.

I don't think this is possible with the current permissions.

[zocker-160]

seems like I can write to that folder, but it is not persisting, I feel like this could cause issues.

$ flatpak run --command=sh net.lutris.Lutris 
[📦 net.lutris.Lutris ~]$ mkdir .var/app/org.openwinecomponents.umu.umu-launcher
[📦 net.lutris.Lutris ~]$ echo "test" > .var/app/org.openwinecomponents.umu.umu-launcher/test
[📦 net.lutris.Lutris ~]$ cat .var/app/org.openwinecomponents.umu.umu-launcher/test 
test
[📦 net.lutris.Lutris ~]$ exit
$ flatpak run --command=sh net.lutris.Lutris 
[📦 net.lutris.Lutris ~]$ cat .var/app/org.openwinecomponents.umu.umu-launcher/test
cat: .var/app/org.openwinecomponents.umu.umu-launcher/test: No such file or directory
[📦 net.lutris.Lutris ~]$ 

[Mershl]

see flathub/net.lutris.Lutris#440 (comment)
a short-term workaround has been implemented in flatpak lutris