hive-priv: require exactly one nix build output path, don't just trim

hive-priv/src/main.rs

@@ -816,10 +816,22 @@ async fn nix_build_toplevel(name: &str, mut writer: Option<&mut OwnedWriteHalf>)
             stderr_buf.lines().last().unwrap_or("").trim()
         );
     }
-    let path = stdout_buf.trim();
-    if path.is_empty() {
-        bail!("nix build {attr} produced no output path");
-    }
+    // `--print-out-paths` prints one line *per output*, not one line
+    // total: `nix build --no-link --print-out-paths nixpkgs#openssl`
+    // prints two (`…-bin`, `…-man`). `config.system.build.toplevel` is
+    // single-output today, so this is one line in practice — but a bare
+    // `.trim()` would silently hand a multi-line string on to
+    // `--system-path` the day that ever changes, which is the same
+    // corrupted-argument failure this function exists to avoid. Require
+    // exactly one line and error otherwise, so a future multi-output
+    // attr fails loudly here instead of downstream in `nixos-container`.
+    let lines: Vec<&str> = stdout_buf.lines().collect();
+    let [path] = lines[..] else {
+        bail!(
+            "nix build {attr} produced {} output path(s), expected exactly 1: {stdout_buf:?}",
+            lines.len()
+        );
+    };
     Ok(path.to_owned())
 }