preparation.md: should we link to the rarewares.org build of opus-tools?

[tenturestept]

the build from mozilla is from 2018.

the rarewares.org build has:

1. libopus 1.5.2 instead of 1.3
2. modern gcc 14
3. 64-bit
4. opus-tools from git master

[FichteFoll]

cc @rokweom