CI: Ignore MSYS2 CI failures for this older stable-branch

Release branches older than the current stable release are intended
for long-term-supported OS distributions such as Debian, RHEL or
Ubuntu LTS where GLib is part of the OS itself, and therefore an
OS vendor that wants to minimize regression risk in other OS components
might reasonably prefer to backport high-impact bug fixes instead of
upgrading to new GLib branches.

MSYS2 does not fall into that category: it is an add-on to an operating
system that does not contain GLib as OS functionality (namely, Windows),
making its maintainers less change-averse than LTS distribution vendors,
and in reality it has already upgraded its packaged GLib to 2.80.x.

Making these CI jobs non-blockers ensures that security fix releases
for LTS Linux distributions do not get held back by build failures on
a platform that is no longer interested in the relevant branch.

Signed-off-by: Simon McVittie <smcv@debian.org>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index ef566e3..77c6bc9 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -329,6 +329,7 @@
       - _build/gobject/libgobject-2.0-0.dll
 
 msys2-mingw32:
+  allow_failure: true
   extends: .only-default
   stage: build
   tags:
@@ -360,6 +361,7 @@
       - _coverage/
 
 msys2-clang64:
+  allow_failure: true
   extends: .only-schedules
   stage: build
   tags: