switch to homegrown pinning

meat.nu

@@ -48,6 +48,11 @@ def nix-build-nom [out: string, source: string, attr: string, extras: list<strin
   } else {
     ^nix-build --log-format internal-json --out-link $out $source -A $attr ...$extras out+err>| ^$env.MONITOR --json
   }
+  # Piping to MONITOR masks nix-build's exit code, so detect failure by the
+  # absence of the out-link.
+  if not ($out | path exists) {
+    error make { msg: "nix-build produced no output" }
+  }
 }
 
 def differ-step [build: string] {