SteamVR setup fails when trying to set capabilities on the steamvr-launcher binary

**Flatpak info**
Flatpak 1.12.7

**Problem description**
When setting up SteamVR, the process tries to set capabilities on the `steamvr-launcher` binary but fails to do so because it requires sudo/root for that. This can be worked around from the host system:

```
sudo setcap CAP_SYS_NICE+ep ~/.var/app/com.valvesoftware.Steam/data/Steam/steamapps/common/SteamVR/bin/linux64/vrcompositor-launcher
```

However it would of course be great to be able to have this working out-of-the-box.

After the workaround SteamVR seems to work great (as far as can work "great" on Linux that is, no audio and annoying bugs like https://github.com/ValveSoftware/SteamVR-for-Linux/issues/334 that make the experience anything but perfect, but these are SteamVR on Linux problems, not Flatpak related) and I can launch and play at least Beat Saber through Proton.

This issue is split off #636.

**Does this issue reproduce with native Steam**
No, as SteamVR can just use sudo there.