CI: Allow failure of windows tests

This is temporary and should be reverted ASAP.
There's currently an unknown libvlc issue with the win32/64
implementations of the thumbnailer. This issue is being investigated
but shouldn't block all our pipelines.

Failures under windows pipelines shouldn't be ignored and it is better
not to disable the thumbnailer tests, so lets switch the failing jobs to
warnings and check them manually when they fail...