Sphinx documentation jobs fail on fork with no version tag

Since commit 8619b5dd ("ci: build & store windows installer") the MinGW jobs fail on forks with no versioned tag:

$ if grep -q "EXESUF=.exe" config-host.mak; then make installer; version="$(git describe --match v[0-9]*)"; mv -v qemu-setup*.exe qemu-setup-${version}.exe; fi
fatal: No names found, cannot describe anything.
ERROR: Job failed: exit code 1

I.e.: https://gitlab.com/legoater/qemu/-/jobs/1553031651#L4010