Use effective uid to set the userHome variable

[amatas]

Summary

Use the function isRootUser to set the userHome variable if root is the effective user in Linux platform. The $USER environment variable doesn't match the effective user of the process in some cases.

Related to #2123

Test plan

As root, run yarn as an unpriviledged user using sudo command:

[root@fedora nodejs-project]# sudo -u vagrant yarn install
yarn install v0.18.0-20161202.0739
[1/4] Resolving packages...
[2/4] Fetching packages...
[3/4] Linking dependencies...
[4/4] Building fresh packages...
Done in 7.77s.

• Use isRootUser function instead USER environment variable
• Fix Running Yarn as a user using sudo crashes due several file permissions denied #2123