Make the CA tests actuall test CA derivations

Fix a mistake in config.nix that was preventing
`NIX_TESTS_CA_BY_DEFAULT` from having any meaningful effect

tests/config.nix.in

@@ -22,6 +22,6 @@ rec {
       builder = shell;
       args = ["-e" args.builder or (builtins.toFile "builder-${args.name}.sh" "if [ -e .attrs.sh ]; then source .attrs.sh; fi; eval \"$buildCommand\"")];
       PATH = path;
-    } // removeAttrs args ["builder" "meta"])
+    } // caArgs // removeAttrs args ["builder" "meta"])
     // { meta = args.meta or {}; };
-} // caArgs
+}