Use validate flavour rather than devel2 for DEBUG CI job
This also builds stage2 with optimisations and -dcore-lint which devel2 doesn't do.
There was some recent breakage on the master branch where -dcore-lint failed.
This also builds stage2 with optimisations and -dcore-lint which devel2 doesn't do.
There was some recent breakage on the master branch where -dcore-lint failed.