appveyor: Workaround a Cygwin setup bug

Run Cygwin's setup a second time to workaround for it's defective
handling of obsolete packages.

(Specifically, python3-lxml is currently obsoleted by python36-lxml, but
that doesn't get installed on the first run)
4 jobs for master in 5 minutes and 46 seconds
Status Job ID Name Coverage
  Docker Image
passed #411440
debian-testing

00:00:33

 
  Build And Test
passed #411441
autotools-build-and-test

00:05:13

passed #411442
meson-build-and-test

00:01:29

 
  External
passed continuous-integration/appveyor/branch