fix builds, update plugins, add plugins

[sbruder]

All plugins should work for most things, though it is possible that there is a malfunction that I did not catch during my tests. Some things that do not work:

• scxvid segfaults due to an incompatibility with xvidcore ≤ 1.3.5 and binutils ≥ 2.31. I submitted a PR to nixpkgs that updates xvidcore (https://github.com/NixOS/nixpkgs/pull/124006) to fix this problem.
• lvsfunc makes vapoursynth.withPlugins fail to evaluate, because it depends on functools32, which evaluates to null on python3. I changed the dependency resolver for the withPlugin interface in https://github.com/NixOS/nixpkgs/pull/123253 to fix this.
• placebo.Deband yields weird artifacts like this that I could not fix, but its other functions seem to work, so I still included it.

Since rgsf segfaults or aborts with malloc(): unaligned tcache chunk detected, I decided not to include it in this PR. The package expression is available on the rgsf branch. It is a dependency of lvsfunc and debandshit, though they work fine except for not being able to degrain 32-bit float formats (for which rgsf would be needed).

Also, because many plugins have no license and therefore is unfree, I added config.allowUnfree = true to the nixpkgs import in the flake, because otherwise they could not be used due to the pure evaluation of flakes. This only has an effect when installing the packages through the flake’s package attribute, not when using the overlay.

[tadeokondrak]

Updating my copy of nixpkgs fixed both issues. Thanks for all this work!