Reduce build/test parallelism in build_tools/rocksdb-lego-determinator (#9788)

Summary:
build_tools/rocksdb-lego-determinator is to generate commands for continuous tests. Recently it changed to by default run tests in parallel with parallelism to be number of CPU processors. This sometimes causes out of space when running so many tests in parallel. Reduce the parallelism by half to temporarily work it around.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9788

Test Plan: Run build_tools/rocksdb-lego-determinator and watch generated commands.

Reviewed By: pdillinger

Differential Revision: D35327704

fbshipit-source-id: 95a8c51a111bb6ab62c456c74ab9c905b457ea8f

build_tools/rocksdb-lego-determinator

@@ -84,9 +84,9 @@ UPLOAD_DB_DIR="
     ]
 }"
 
-# set default RATIO to 1, which sets J=$(nproc) and j=$(nproc)
+# set default RATIO to 2, which sets J=$(nproc)/2 and j=$(nproc)/2
 if [ -z $RATIO ]; then
-  RATIO=1
+  RATIO=2
 fi
 
 # Should probably be called PARALLEL_TEST