(ziclh lang rust): use compile-time path for yj

This will have to be replaced with a small Rust program, but I don't
want to resolve the chicken-egg problem right now.

lang/rust/default.nix

@@ -1,4 +1,4 @@
-{ chickenPackages, libsodium, callPackage, xxd }:
+{ chickenPackages, libsodium, callPackage, xxd, yj }:
 (callPackage ../../lib/build-chicken-parallel {}) {
   name = "zilch-lang-rust";
   src = ./.;
@@ -14,6 +14,8 @@
 
     xxd
   ];
+
+  overrides.NIX_CFLAGS_COMPILE = "-DYJ_PATH=\"${yj}/bin/yj\"";
   overrides.preBuild = ''
     (cat ${./helpers/src/bin/buildscript-runner.rs}; printf '\0') | xxd -i -n runner_source > runner_source.h
     (cat ${./helpers/src/bin/rustc-wrapper.rs}; printf '\0') | xxd -i -n rustc_wrap_source > rustc_wrap_source.h