firefox-nightly fails to use vaapi with mesa's radeon driver on NixOS

[colemickens]

see: https://github.com/NixOS/nixpkgs/issues/156126

Describe the bug

❯ firefox             

(firefox:1202676): Gtk-WARNING **: 12:37:57.658: Theme file for capitaine-cursors-white has no directories
libva info: VA-API version 1.13.0
libva info: Trying to open /run/opengl-driver/lib/dri/radeonsi_drv_video.so
libva info: Trying to open /usr/lib/dri/radeonsi_drv_video.so
libva info: Trying to open /usr/lib32/dri/radeonsi_drv_video.so
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/radeonsi_drv_video.so
libva info: Trying to open /usr/lib/i386-linux-gnu/dri/radeonsi_drv_video.so
libva info: va_openDriver() returns -1

yet:

❯ vainfo
libva info: VA-API version 1.13.0
libva info: Trying to open /run/opengl-driver/lib/dri/radeonsi_drv_video.so
libva info: Found init function __vaDriverInit_1_13
libva info: va_openDriver() returns 0
vainfo: VA-API version: 1.13 (libva 2.13.0)
vainfo: Driver version: Mesa Gallium driver 21.3.3 for AMD RENOIR (DRM 3.44.0, 5.16.0, LLVM 13.0.0)
vainfo: Supported profile and entrypoints
      VAProfileMPEG2Simple            :	VAEntrypointVLD
      VAProfileMPEG2Main              :	VAEntrypointVLD
      VAProfileVC1Simple              :	VAEntrypointVLD
      VAProfileVC1Main                :	VAEntrypointVLD
      VAProfileVC1Advanced            :	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointEncSlice
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264Main               :	VAEntrypointEncSlice
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointEncSlice
      VAProfileHEVCMain               :	VAEntrypointVLD
      VAProfileHEVCMain               :	VAEntrypointEncSlice
      VAProfileHEVCMain10             :	VAEntrypointVLD
      VAProfileHEVCMain10             :	VAEntrypointEncSlice
      VAProfileJPEGBaseline           :	VAEntrypointVLD
      VAProfileVP9Profile0            :	VAEntrypointVLD
      VAProfileVP9Profile2            :	VAEntrypointVLD
      VAProfileNone                   :	VAEntrypointVideoProc

This is using latest firefox nightly from flake-firefox-nightly but it's using the same packaging that nixpkgs's bin packaging uses.

This is with nixos-unstable current: 6d8215281b2f87a5af9ed7425a26ac575da0438f