cirrus.yml: Fix the MSYS2 task

The MSYS2 task in the Cirrus-CI is currently failing with error messages
like this:

 warning: database file for 'ucrt64' does not exist (use '-Sy' to download)
 :: Starting core system upgrade...
  there is nothing to do
 :: Starting full system upgrade...
 error: failed to prepare transaction (could not find database)

Seems like it can be fixed by switching to a newer release and by refreshing
the database one more time after changing the /etc/pacman.conf file.

Message-Id: <20210504100223.25427-30-alex.bennee@linaro.org>
Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>