GHA: Pin version of CMake for MinGW build

GHA: Pin version of CMake for MinGW build

Turns out that "apt install cmake" is useless since
Github installs also CMake but not as a package. So
make sure we control which CMake we get and not
Github.

Switch to get-cmake action (which also installs ninja)
and remove the installation via apt.

Backport from master but much more limited since we
do not have Android nor AWS LC builds in release/2.6.

Change-Id: Ief11d5d7ccae084c863b50054d4e97c71f57d6f0
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250401170420.29808-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31299.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit da65c4fdde20e36df38a90c1f1ef1d8ebf486c28)
Message-Id: <20250402153250.4764-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/search?l=mid&q=20250402153250.4764-1-gert@greenie.muc.de
Signed-off-by: Gert Doering <gert@greenie.muc.de>