CI: enforce packages upgrade for Msys2 on Windows

We need to install libgnutls-devel package to build the host tool,
mkeficapsule, and as of now, there seems to be a depencency conflict
in the current msys2 installer;

   :: installing libp11-kit (0.24.1-1) breaks dependency \
	'libp11-kit=0.23.22' required by p11-kit

To resolve this conflict, however, the initial "pacman -Syyuu" in
'tools_only_windows' job is not enough. Another "pacman -Su" will
enforce all the out-of-date packages being upgraded.
(Probably the first "-Syyuu" can be changed to "-Syu".)

See the installation steps in
  https://www.msys2.org/



Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>