Fix for installation of desktop file

Continues  #2630
Fixes #2647
Installs desktop file with respect to `INSTALL_PREFIX`.
* `sudo script/grunt install` will install `Atom.desktop` to `/usr/local/share/applications` instead of `/usr/share/applications/`
* When a custom `INSTALL_PREFIX` is set the user will have to install the desktop file manually by copying `$INSTALL_PREFIX/share/applications/Atom.desktop` to `/usr/local/share/applications`
* Fix when `INSTALL_PREFIX` ends with `/`
The check for `/tmp` is important. 
`mkdeb` creates a temporary folder with `mktemp` ( which returns `/tmp` or `$TMPDIR` ), then calls `install` task to install everything into this folder and then creates a debian package from the folder structure.
Without a check for `/tmp` `install` task would create a desktop file whose `Exec` and `Icon` items point to something like `/tmp/somefolder/share/atom/atom`, which is wrong because that is not the installation path of atom. When running `mkdeb` task the desktop file should point to `/usr/share/atom`.