snap builds in cleanbuild but fails on buildd

Snap for corebird hosted in my GitHub repo builds perfectly fine on my PC using `snapcraft cleanbuild` but fails on the buildd system at build.snapcraft.io. Specifically on the buildd it complains about a file which it should be downloading from ftp.gnome.org not existing.

from the build log at https://build.snapcraft.io/user/diddledan/corebird-snap/38228:

/bacfbbd4b618017965cf05163fa2626f-xenial/parts/corebird/jhbuild/usr/lib/python2.7/site-packages/modulesets/http:/ftp.gnome.org/pub/GNOME/teams/releng/3.25.1/gnome-apps-3.25.1.modules: [Errno 2] No such file or directory: u'/bacfbbd4b618017965cf05163fa2626f-xenial/parts/corebird/jhbuild/usr/lib/python2.7/site-packages/modulesets/http:/ftp.gnome.org/pub/GNOME/teams/releng/3.25.1/gnome-apps-3.25.1.modules'

The file should be downloaded from http://ftp.gnome.org/pub/GNOME/teams/releng/3.25.1/gnome-apps-3.25.1.modules. I am unsure whether the URL has somehow become malformed in jhbuild so that it thinks it's a file path or whether it really does try to download the URL into the path /bacfbbd4b618017965cf05163fa2626f-xenial/parts/corebird/jhbuild/usr/lib/python2.7/site-packages/modulesets/http:/ftp.gnome.org/pub/GNOME/teams/releng/3.25.1/gnome-apps-3.25.1.modules'

My snap is hosted on GitHub at https://github.com/diddledan/corebird-snap