use pipe sleep_strategy for the client

pipe performs generally better than vanilla for network io.

bench/emper.py

@@ -459,8 +459,8 @@ def prepare_client(data_dir: Path, fetch=True):
     print("Preparing the client ...")
     if not EMPER_CLIENT_DIR.exists():
         checkout_worktree(EMPER_CLIENT_DIR, client_checkout)
-    build_emper(EMPER_CLIENT_DIR, '')
-    # build_emper(EMPER_CLIENT_DIR, '-Dworker_sleep_strategy=pipe')
+    #build_emper(EMPER_CLIENT_DIR, '')
+    build_emper(EMPER_CLIENT_DIR, '-Dworker_sleep_strategy=pipe')
 
 
 def repo_fetch():