Since c378a33 we have a patch to add svt-av1 support to
ffmpeg-full. The default AV1 encoder is still libaom but svt-av1 has a
higher priority than rav1e (see libavcodec/allcodecs.c). And since
svt-av1 is much faster than rav1e (which in turn should be faster than
libaom) we don't really need rav1e support by default anymore (I guess
it can be useful but overall svt-av1 should currently be the best
option).
And because rav1e's build did break a few times in the past and needs a
lot of Rust dependencies (including cargo-c to make it C-ABI
compatible) it might be best to simply disable it for now.

Benchmarks (though it is difficult to compare them that way):
- https://openbenchmarking.org/test/pts/svt-av1
- https://openbenchmarking.org/test/pts/rav1e
- https://openbenchmarking.org/test/pts/aom-av1

I've merged my local package definitions with the ones in Nixpkgs.

ffmpeg-full: disable rav1e by default (in favor of svt-av1)