gitlab: fix incorrect build-tools path being used on macOS
The relevant CI log
dyld: Library not loaded: /Users/gst-ci/builds/gstreamer/cerbero/cerbero-build/dist/darwin_universal/x86_64/lib/liborc-0.4.0.dylib
Referenced from: /Users/gst-ci/builds/xhaakon/gstreamer-sharp/cerbero-build/dist/darwin_universal/x86_64/bin/orcc
Reason: image not found
Edited by Matthew Waters