ci: Build with --disable-debug in distcheck stage

In addition to the --enable-debug=werror (i.e. full + -Werror) of the build stage, so we should compile all code paths (relative to debug mode). This requires projects not to use DISTCHECK_CONFIGURE_FLAGS directly in their Makefile.am, but AM_DISTCHECK_CONFIGURE_FLAGS, so that the flags we set here are not overwritten.