Add back par_style = xar to benchmark builds

Summary: The benchmark server currently complains about not being able to run `_compile_tool`. This is probably because the `par_style = "xar"` line was removed for `benchmark.server` in D26929344. It think we need it for the client / server builds which package tools that are run externally, but should be fine to not have it for `upload_to_phab` and `format_results`.

Based on Facebook D26983654

benchmarks/TARGETS

···
         "server.py",
     ],
     main_module = "pyro.benchmarks.server",
+    par_style = "xar",
     deps = [
         ":run",
         ":server_handler",
···
         "client.py",
     ],
     main_module = "pyro.benchmarks.client",
+    par_style = "xar",
     deps = [
         ":fbpkg_handler",
         ":rpc-py3-clients",