Use tmpfs for PeerTube Runner

[emansom]

Describe the problem to be solved

Currently the spawned ffmpeg process by the PeerTube runner writes a lot to the physical storage backed ~/.cache directory.

This wears out the hardware unnecessarily.

Describe the solution you would like

On systems with sufficient (>= 16GB) free available system memory, for the PeerTube runner to use the XDG provided tmp directory instead.

This'd lessen the wear of physical storage during transcoding.