This is a fix for https://github.com/adobe/brackets/issues/5513

Some versions of Linux don't have TMPDIR, TMP or TEMP defined.
Installing an extension relies on the existence of a temp folder
location.
Since we are using node-temp (https://github.com/bruce/node-temp) to
determine a temp dir location, the node module will resolve to ~/tmp if
none of the environment variables exists. If this directoruy doesn't
exist, then the install operation will fail.