(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/src/registry.sld

@@ -15,9 +15,11 @@
    lockfile-entry? lockfile-entry-name lockfile-entry-version lockfile-entry-source lockfile-entry-checksum lockfile-entry-dependencies)
 
  (begin
+   (define yj-path (foreign-value "YJ_PATH" nonnull-c-string))
+
    ;; Shell out to a TOML-to-JSON parser. This will be replaced with a Nix-native solution later(tm).
    (define (parse-toml toml-to-parse)
-     (define-values (read-port write-port pid) (process "yj" '("yj" "-tj")))
+     (define-values (read-port write-port pid) (process yj-path '("yj" "-tj")))
      (write-string toml-to-parse write-port)
      (close-output-port write-port)
      (define parsed (json-read read-port))