_v2-entry-point: Make sure ./var is not a symlink

Some users might be tempted to redirect ./var to another filesystem, but this will make the runtime a lot slower, more space-consuming and less reliable, because we rely on being able to make hard links between the distributed files and the temporary container root.

The correct way to relocate the Steam Linux Runtime depot is to set up a Steam library on a suitable filesystem (which for best results should be a fully-featured Unix filesystem like ext4, btrfs or xfs, although NTFS can be made to work with some difficulty), and then use Steam's "Properties -> Local Files -> Move install folder..." to move the Steam Linux Runtime into it.

Helps: https://github.com/ValveSoftware/steam-runtime/issues/495