Ok I just added the plugin folder paths to my DAW's preferences directly. This is probably suboptimal (or should I say "subelegant"), but it works.
Of course I had no idea where the plugin folders might be, because NixOS doesn't put them in the usual locations¹ (~/.vst/ and so on), but I ended up figuring out they're `/etc/profiles/per-user/myusername/lib/vst` (and so on)
¹ I'm not going to blame the project for that, having stuff in non-standard locations (but still working fine!) is part of the NixOS contract. I whish the documentation were more explicit about those nonstandard locations, though.