[sb2] Fix can_accelerate_rpm under new lua. Fixes JB#54116

The (first) return value of os.execute has changed between lua 5.1
and 5.2. On old lua, successful execution returned 0, failure
(exit 1 in script) returned 256. On new lua, successful execution
returns true and failure false.

scratchbox2/modes/obs-rpm-install/fs_rules.lua

@@ -85,7 +85,7 @@ function can_accelerate_rpm()
     ]=]
 
 	local rc = os.execute(string.format(test, tools, target_root))
-	return rc == 0
+	return rc
 end
 
 if can_accelerate_rpm() then