[infer][buck] run Infer in single core on Buck projects

Summary: Fixing the support for single core analysis in 8ce79a0613 was improving the performance of the genrule-based integration by more that 2X. Running Infer in single core more with the external compiler integration for Buck also improves the performance significantly.

Reviewed By: mbouaziz

Differential Revision: D7989255

fbshipit-source-id: 71fb842

infer/lib/python/inferlib/bucklib.py

@@ -49,7 +49,7 @@ def prepare_build(args):
     configuration that tells buck to use that script.
     """
 
-    infer_options = ['--buck']
+    infer_options = ['--buck', '--jobs', '1']
 
     if args.java_jar_compiler is not None:
         infer_options += [