Set the executable bit on the Discord executable in Linux versions

In the Linux distributions of Discord, the executable bits aren't set in either the Discord or DiscordCanary tar files. As a result, the first time you go to run Discord, it will tell you that you don't have permission to run it. We can set the bit manually using `chmod +x` on the terminal, but it _should_ be set in the tar file so we don't have to do this.

This should be a one-line fix in a build configuration somewhere.