Fix for Linux action manifest load failing

[OwlchemyDawson]

Corrects the UserProjectSettings::SetUserDefinedSettings logic to properly replace backslashes with forward slashes in the settings.actionManifestPath string before attempting to determine if the file exists.

[cklarhorst]

Thanks. This PR helped me to get steamvr working in Unity.

Sadly, it was not enough:
std::string fullPath = UserProjectSettings::GetCurrentWorkingPath() + "\\Assets\\" + settings.actionManifestPath;
needed to be changed to:
std::string fullPath = UserProjectSettings::GetCurrentWorkingPath() + "/Assets/" + settings.actionManifestPath;

Additionally, the Unity 2021.3 editor still crashes with default settings, but setting "Mirror View Mode" to "None" and "Stereo Rendering Mode" to "Multi Pass" fixes that. (The game view is sadly black, but VR is working).