cygwin CI: work around gcovr bug
cygwin CI: work around gcovr bug

Due to https://github.com/gcovr/gcovr/pull/576 it is not possible to
`pip install gcovr` and have it work.

It is possible, but not ideal, to install the cygwin gcovr package,
which is unmaintained and built for python36 while only depending on
python39.

This is of course not a problem on the other CI jobs, where we either
install it from a distro repository that ensures it is stable and
backports patches, or simply tests that the resulting image passes tests
before baking it as a CI images update.

gcovr upstream isn't sure when they are going to release a new version
that fixes this bug. There is a new feature release scheduled "soon".

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