CI: Mark Archlinux/x86 as allowing failures

Archlinux is a rolling distro.  As a consequence, rebuilding the container
periodically changes the toolchain, and this affects all stable branches in
one go.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>
Release-Acked-By: Oleksii Kurochko <oleksii.kurochko@gmail.com>

automation/gitlab-ci/build.yaml

@@ -573,11 +573,13 @@ archlinux-gcc:
   extends: .gcc-x86-64-build
   variables:
     CONTAINER: archlinux:current
+  allow_failure: true
 
 archlinux-gcc-debug:
   extends: .gcc-x86-64-build-debug
   variables:
     CONTAINER: archlinux:current
+  allow_failure: true
 
 centos-7-gcc:
   extends: .gcc-x86-64-build