Use a different method to build the generate rule

This one seems to work both here and on CI.

test/Makefile

@@ -312,10 +312,7 @@ core-app-generate-erl-include: build clean-core-app-generate-erl-include
 
 	$i "Append rules to the Makefile to generate a .erl module"
 	$t echo "\$$(PROJECT).d:: src/generated.erl" >> $(APP)/Makefile
-	$t echo "src/generated.erl:: script.sh;" \
-		"printf \"%s\\n\"" \
-			"\"-module(generated).\"" \
-			"\"-include(\\\"included.hrl\\\").\" > \$$@" >> $(APP)/Makefile
+	$t echo "src/generated.erl:: script.sh; echo \"-module(generated).\" > \$$@; echo \"-include(\\\"included.hrl\\\").\" >> \$$@" >> $(APP)/Makefile
 
 	$i "Generate the .hrl file"
 	$t mkdir $(APP)/include/