Disable FBOSS OSS tests in on-diff jobs (#1924)

Summary:
X-link: facebook/folly#1924

FBOSS OSS on-diff job is failing due to random test failures.

Disabling the tests for now to avoid build breakages.

Reviewed By: shri-khare

Differential Revision: D42550176

fbshipit-source-id: 40249f3d2d05e695bb6f00e1fea5abc36265226d

build/fbcode_builder/manifests/fboss

@@ -45,3 +45,6 @@ nlohmann-json
 fbcode/fboss/github = .
 fbcode/fboss/common = common
 fbcode/fboss = fboss
+
+[sandcastle]
+run_tests = off