Builds from master fail on Jenkins mingw builder

On https://jenkins.torproject.org/view/tor/ the builds from git master for tor-ci-mingwcross-master builder are failing for a long time (Last successful build (# 2226), 2 mo 20 days ago )

A failing build log e.g. https://jenkins.torproject.org/view/tor/job/tor-ci-mingwcross-master/lastCompletedBuild/ARCHITECTURE=amd64,SUITE=stretch/consoleFull#-16059184715da2eb1d-1267-4376-8b22-f5f143383dc7

21:28:05 + cp src/test/test-child.exe /srv/jenkins-workspace/workspace/tor-ci-mingwcross-master/ARCHITECTURE/amd64/SUITE/stretch/RESULT/bin/
21:28:05 cp: cannot stat 'src/test/test-child.exe': No such file or directory
21:28:05 + rc=1

There are no usable artifacts as a result, for those who wish to use them. Aside, no artifacts are kept on Appveyor too.

Trac:
Username: harig