ffmpeg-headless: enable additional encoders by default

This adds some extremely helpful and popular encoders in by default:
* openjpeg
* celt
* libwebp
* libaom

On the `master` branch, closure size for ffmpeg-headless went up 18.5 MiB.
```
$ nix store diff-closures nixpkgs#ffmpeg-headless^bin .#ffmpeg-headless^bin
celt: ∅ → 0.11.3, +168.4 KiB
ffmpeg-headless: +70.0 KiB
giflib: ∅ → 5.2.2, +398.7 KiB
lcms2: ∅ → 2.16, +466.2 KiB
lerc: ∅ → 4.0.0, +840.2 KiB
libaom: ∅ → 3.9.0, +8047.8 KiB
libdeflate: ∅ → 1.20, +427.0 KiB
libtiff: ∅ → 4.6.0, +655.9 KiB
libvmaf: ∅ → 3.0.0, +2665.0 KiB
libwebp: ∅ → 1.4.0, +2559.7 KiB
openjpeg: ∅ → 2.5.2, +1525.1 KiB
zstd: ∅ → 1.5.6, +1158.0 KiB

$ nvd diff $(nix build nixpkgs#ffmpeg-headless^bin --print-out-paths --no-link) $(nix build .#ffmpeg-headless^bin --print-out-paths --no-link)
<<< /nix/store/4n60lnj3zkjpasd4c56bzhpx2m8lc1sx-ffmpeg-headless-6.1.1-bin
>>> /nix/store/884f487w5hac6rs94jq6hq5zqkxdv666-ffmpeg-headless-6.1.1-bin
Added packages:
[A.]  #01  celt        0.11.3
[A.]  #02  giflib      5.2.2
[A.]  #03  lcms2       2.16
[A.]  #04  lerc        4.0.0
[A.]  #05  libaom      3.9.0
[A.]  #06  libdeflate  1.20
[A.]  #07  libtiff     4.6.0
[A.]  #08  libvmaf     3.0.0
[A.]  #09  libwebp     1.4.0 x2
[A.]  #10  openjpeg    2.5.2
[A.]  #11  zstd        1.5.6
Closure size: 66 -> 78 (15 paths added, 3 paths removed, delta +12, disk usage +18.5MiB).
```

0 files changed, 0 insertions, 0 deletions