CI: produce nightly builds on merge requests only

VLCKit is quite static with no dynamic changes without merge requests and a fixed libvlc hash that does not change without a merge requests.

Hence, until this MR, there were binaries created every night running for many hours and thereby blocking the builders resulting in the exact same binary as the night before.

If this MR is accepted, I intend to impose the same limit on the 3.0 branch VLCKit.