Allow configuration of used driver via ENV variable

[Shadowghost]

Currently our shipped vainfo or any other program using our ffmpeg can't explicitly set the used driver via the ENV var. I don't see a reason why we should explicitly prevent this, so I added it back.

To explicitly set this environment for Jellyfin only (e.g. multiple GPUs, one for transcoding one for something else) the variable was renamed from LIBVA_DRIVER_NAME to LIBVA_DRIVER_NAME_JELLYFIN.

[nyanmisaka]

On a Nvidia prime laptop with both nvidia_dri_video and iHD_dri_video driver installed, if user set LIBVA_DRIVER_NAME=nvidia explicitly to global system to enable hw decoding in firefox/chrome, then the jellyfin-ffmpeg cannot initialize iHD driver for the integrated iGPU/APU to transcoding.

You can rename it to something like LIBVA_DRIVER_NAME_JELLYFIN.

[Shadowghost]

Sounds like a reasonable approach for this kind of situations to me. I changed the code in regards to this.