Adding a custom version that uses file:/// as the protocol doesn't work with Proxied asset downloading

[CakeLancelot]

Tested on Linux with Proton 10, not sure if this affects Windows.

To reproduce:

1. Have a local folder with all .resourceFiles and .unity3d's for a specific game build
2. Go to Settings -> Game Builds, then click Add Build
3. Switch to the Add Manually tab and enter in folder from step 1 in file:/// format
4. Ensure proxied asset downloading is on and attempt to launch game (main.unity3d itself will load, but all additional assets will fail)