Docs: Mention unofficial builds on Nix

Author: cameel

docs/installing-solidity.rst

@@ -102,6 +102,8 @@ Another script can be used to install the official binary:
     are produced and maintained by users and not vetted in any way by the distro maintainers.
     Exercise caution when using them.
 
+On Nix, unofficial builds are available via `solc.nix<https://github.com/hellwolf/solc.nix>`_.
+
 Ubuntu packages for versions up to 0.8.30 are available in the
 [`ethereum/ethereum` PPA](https://launchpad.net/~ethereum/+archive/ubuntu/ethereum).
 However, we have discontinued this distribution method and future versions will not be added there.