ci: Add MBUILD_TARGET for NetBSD and OpenBSD

Commit b2bdb972c0 added MBUILD_TARGET to ensure that meson builds
the tests before running them, this adds MBUILD_TARGET to OpenBSD
and NetBSD builds as well where it was missing.

No backpatching since OpenBSD and NetBSD support does not exist
in the backbranch CI.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAN55FZ2LNnRrtL+cpSdEg44fQcLPq_GjJjfNa0vz+xqEdq=ZHw@mail.gmail.com

.cirrus.tasks.yml

@@ -322,7 +322,7 @@ task:
         build
     EOF
 
-  build_script: su postgres -c 'ninja -C build -j${BUILD_JOBS}'
+  build_script: su postgres -c 'ninja -C build -j${BUILD_JOBS} ${MBUILD_TARGET}'
   upload_caches: ccache
 
   test_world_script: |