Startmovie doesn't work with webm format on Linux

[meanwhile131]

In TF2, i can convert demo files (.dem) to video by typing:

sv_cheats 1
host_framerate 60
startmovie output webm/jpeg/tga/raw
playdemo demofile

And waiting. Previously (before the 64 bit update) this worked well with all startmovie options, but now i get: Failed to launch startmovie! in console when using webm as an output format. No webm file is created if i play the demo using playdemo demofile.

AVI and H264 aren't supported by TF2 on Linux, so I am told to use WEBM, which fails. I know that i can use raw and convert it to video with ffmpeg, but that generates 100s of GB of images, so i would like to use the native feature.

How to reproduce:

1. Launch TF2
2. Go into a server (map ctf_2fort for example)
3. record demofile in console to start recording a demo
4. stop to stop recording
5. Go into the main menu (disconnect)
6. sv_cheats 1
7. host_framerate 60
8. startmovie output webm
9. playdemo demofile
10. Wait until the demo is finished or you want to stop recording
11. endmovie
12. No file is created (should be output.webm in the tf directory) (You can find more information on the TF2 Wiki)

Operating System Version: "NixOS 24.11 (Vicuna)" (64 bit) Kernel Name: Linux Kernel Version: 6.6.34 X Server Vendor: The X.Org Foundation X Server Release: 12101013 X Window Manager: i3 Steam Runtime Version: steam-runtime_0.20240610.91380

Video Card: Driver: AMD AMD Radeon RX 6600 (radeonsi, navi23, LLVM 17.0.6, DRM 3.54, 6.6.34) Driver Version: 4.6 (Compatibility Profile) Mesa 24.1.1 OpenGL Version: 4.6