use yarn network mutex

Summary: The file system mutex caused some problems in Sandcastle. This diffs switches for the default setting (network mutex) in the hope this fixes the problem

Reviewed By: passy

Differential Revision: D9132069

fbshipit-source-id: 5abb4d33b6db2a5d2b3f0788116ca6941e554591

scripts/yarn-install.js

@@ -39,7 +39,7 @@ Promise.all(
           new Promise((resolve, reject) => {
             const cwd = pkg.replace('/package.json', '');
             exec(
-              [YARN_PATH, '--mutex', 'file'].join(' '),
+              YARN_PATH,
               {
                 cwd,
               },