macOS: RPATH management needs further repair

Looks like there's yet another corner case missing:

dyld[65439]: Library not loaded: @rpath/libgstreamer-1.0.0.dylib
  Referenced from: <1AFFAB85-3611-3F4F-9096-DC8047D723D0> /private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/gstreamer-1.0/gst-plugin-scanner
  Reason: tried: '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/lib/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/lib/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/gstreamer-1.0/../libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/gstreamer-1.0/../../../lib/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/gstreamer-1.0/../lib/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/lib/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/lib/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/gstreamer-1.0/../libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/gstreamer-1.0/../../../lib/libgstreamer-1.0.0.dylib' (no such file), '/private/tmp/builds/nirbheek/cerbero/cb/dist/darwin_universal/libexec/gstreamer-1.0/../lib/libgstreamer-1.0.0.dylib' (no such file)

https://gitlab.freedesktop.org/nirbheek/cerbero/-/jobs/65668725