Get MacOSX10.14.sdk.tar.xz
from the following site and convert it to MacOSX10.14.sdk.tgz
.
https://github.com/phracker/MacOSX-SDKs/releases
Login to the VM and:
mkdir -p gitian-builder/inputs
mv MacOSX10.14.sdk.tar.gz gitian-builder/inputs