ci: windows: run tests under timeout.

ci: windows: run tests under timeout.

On windows ci/cfbot currently regularly hangs / times out. Presumably this is due
to the issues discussed in
https://postgr.es/m/CA%2BhUKG%2BG5DUNJfdE-qusq5pcj6omYTuWmmFuxCvs%3Dq1jNjkKKA%40mail.gmail.com
which lead to reverting 75674c7ec1b everywhere but master.

But it's hard to tell - because the entire test task times out, we don't get
to see debugging information.

This commit adds a timeout (using git's timeout binary) to all vcregress
invocations, which should address the problem for now. It might also be a good
idea to add timeouts to the other operating systems at a later time.

The diff is a bit larger than one might think necessary: Yaml doesn't like % -
from the windows command variable syntax - at the start of an unquoted
string...

Discussion: https://postgr.es/m/20220110005704.es4el6i2nxlxzwof@alap3.anarazel.de