release: GCE builds depend on ftp

GCE images are required by Google to include their source code; we do
this by extracting Missing repository, expected "{src repository:path ...}" or "{src path repository=...}" in: {src,ports}.txz into the images, from the (legacy)
distribution sets.

Make sure those distribution sets actually exist.

Reviewed by: ivy
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53567