buildstream/_project.py: Restrict max-jobs

even if the machine have mores cores available

Patch taken from YBD: https://gitlab.com/baserock/ybd/blob/master/ybd/app.py#L227

buildstream/_project.py

@@ -572,7 +572,10 @@ class Project():
 
         # Extend variables with automatic variables and option exports
         # Initialize it as a string as all variables are processed as strings.
-        output.base_variables['max-jobs'] = str(multiprocessing.cpu_count())
+        # Based on some testing (mainly on AWS), maximum effective
+        # max-jobs value seems to be around 8-10 if we have enough cores
+        # users should set values based on workload and build infrastructure
+        output.base_variables['max-jobs'] = str(min(multiprocessing.cpu_count(), 8))
 
         # Export options into variables, if that was requested
         output.options.export_variables(output.base_variables)