Support `download-ci-llvm` on NixOS
Support `download-ci-llvm` on NixOS

In particular, the CI built `libLLVM-*.so` needs to have `libz.so`
RPATHed so that binaries like `llvm-config` work at all.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust