Permissions are now changed for custom appImages

[pakeke-constructor]

Previously, if you had a custom appImage, the execute permission wasn't set on your AppImage.

This PR changes/fixes that

[pfirsich]

There is a chance that the we don't have permission to add the executable bit on the custom AppImage in which case this might break for some people. This is probably (like 95%) fine, but could you add a check that the executable bit is not already set? I'd probably do if not os.access(source_appimage, os.X_OK). Would you mind adding that?

[pakeke-constructor]

Yup! sure thing. Good catch btw