pgsql: ci: Add task testing windows with mingw

ci: Add task testing windows with mingw

For now the task has been set to be manually triggered, as we are already
limited by the amount of CI time available for windows, particularly on cfbot.

Author: Melih Mutlu <m.melihmutlu@gmail.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAGPVpCSKS9E0An4=e7ZDnme+y=WOcQFJYJegKO8kE9=gh8NJKQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/967db242c2aa7d50300e20bd8a403bd905feec38

Modified Files
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.cirrus.yml         | 91 +++++++++++++++++++++++++++++++++++++++++++++--------
src/tools/ci/README |  2 +-
2 files changed, 79 insertions(+), 14 deletions(-)