CI: Azure: Rework how we update MSYS2

Based on reading https://www.msys2.org/docs/ci/

 and "Other Systems"
rework how we update MSYS2 to the current version.  We run it once, to
perform nothing other than being the first run, then we run pacman
twice.

Signed-off-by: Tom Rini <trini@konsulko.com>