Use os.tmpdir() insted of /tmp

`/tmp` isn't always available, is on precious [RAM-backed fs that is cleared at boot](https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt) or simply not
what the user has set his `$TMPDIR` to. According to specifications, we
should use `$TMPDIR`, which node lets us find through `os.tmpdir()`.

Also, [contributing.md](https://github.com/atom/atom/blob/master/docs/contributing.md#code-guidelines) isn't in favour of using platform-dependent code.

This commit focuses only on Linux, and leaves OS X as is with `/tmp` for
discussion.