ci: mark the msys job as manual and allowed to fail

the msys setup is not deterministic due to issues with msys installation,
so it keeps breaking every now and then.

Part-of: <gstreamer/orc!48>
5 jobs for win-page-align in 1 minute and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #3853683
debian sid

00:00:33

passed #3853687
gst-macos-10.15
meson macos

00:01:24

manual #3853686
gstreamer-windows 1809 windows docker allowed to fail manual
msys2
passed #3853684
gstreamer-windows 1809 windows docker
vs2017 amd64

00:01:00

passed #3853685
gstreamer-windows 1809 windows docker
vs2017 x86

00:01:00