stages/dnf: don't ignore unavailable repositories

dnf skips unavailable repositories by default, which only leads to
harder understand errors later. Configure it to fail when any of the
passed repositories cannot be reached.

stages/org.osbuild.dnf

@@ -253,6 +253,7 @@ def main(tree, options):
             "--forcearch", basearch,
             "--setopt", "reposdir=",
             "--setopt", f"install_weak_deps={weak_deps}",
+            "--setopt", f"skip_if_unavailable=false",
             "--releasever", releasever,
             "--noplugins",
             "--config", dnfconf