ALVR Crashes when XDG_RUNTIME_DIR is not set.

## Description

When the environment variable `XDG_RUNTIME_DIR` is unset, ALVR will crash at `CEncoder::Run`
See above image for what ALVR _should_ do.

## General Troubleshooting
- [x] I carefully followed the instructions in the [README](https://github.com/alvr-org/ALVR/blob/master/README.md) and successfully completed the setup wizard
- [x] I read the ALVR Wikis [here](https://github.com/polygraphene/ALVR/wiki) and [here](https://github.com/alvr-org/ALVR/wiki)

## Environement

### Hardware
**Note**: for Linux, an upload to the [`hw-probe`](https://linux-hardware.org/) database is preferred: `hw-probe -all -upload`

Linux. I've done the research into ALVR's code already to even determine that this was the issue, so I know no other details are needed.

### Installation
**ALVR Version**: v18.2.3

**SteamVR Version**: latest non-beta at the time of posting this (not applicable to the issue)

**Install Type**:
- [ ] Packaged (`exe`, `deb`, `rpm`, etc)
- [x] Portable (`zip`)
- [ ] Source

**OS Name and Version** (`winver` on Windows or `grep PRETTY_NAME /etc/os-release` on most Linux distributions):
Ubuntu 20.04