[ci] make gitu executable in release assets

[practicalli-johnny]

When building gitu for a new release, set the executable bit on the gitu file which is included in the release assets (Linux and MacOS).

Including an executable library allow the use of DRA to download and install a release asset from the Gitu GitHub repository.

It is not currently possible to use DRA to install Gitu, as no executable file is found in the .zip file of the release (tested on Linux).

Thank you.