Fix garbled output from concurrent threads in uplc-evaluator (#7555)

Set LineBuffering on stdout/stderr in both the service and test suite
to prevent output interleaving when multiple threads write concurrently.
This ensures each line is written atomically.

Author: Unisay

plutus-benchmark/plutus-benchmark.cabal

@@ -924,3 +924,4 @@ test-suite uplc-evaluator-integration-tests
     , temporary
     , text
     , uuid
+    , with-utf8

plutus-benchmark/uplc-evaluator/Main.hs

@@ -36,7 +36,7 @@ import PlutusPrelude (void)
 import System.Directory
 import System.Exit (exitFailure)
 import System.FilePath (takeBaseName, takeExtension, (</>))
-import System.IO (hPutStrLn, stderr)
+import System.IO (BufferMode (LineBuffering), hPutStrLn, hSetBuffering, stderr)
 import UntypedPlutusCore qualified as UPLC
 import UntypedPlutusCore.Evaluation.Machine.Cek qualified as Cek
 import UntypedPlutusCore.Parser qualified as UPLC.Parser
@@ -464,6 +464,7 @@ takeFileName = reverse . takeWhile (/= '/') . reverse
 -- | Main entry point
 main :: IO ()
 main = withUtf8 do
+  hSetBuffering stderr LineBuffering -- Prevent garbled output from concurrent threads
   initializeTime -- Required before using getTime from criterion-measurement
   config <- execParser opts
   hPutStrLn stderr "UPLC Evaluator Service starting..."