fix: explicitly mark AccessorCommandHandler methods as Mutable

Author: My-Name-Is-Jeff

src/main/java/gg/skytils/skytilsmod/mixins/transformers/accessors/AccessorCommandHandler.java

@@ -21,6 +21,7 @@
 import net.minecraft.command.CommandHandler;
 import net.minecraft.command.ICommand;
 import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Mutable;
 import org.spongepowered.asm.mixin.gen.Accessor;
 
 import java.util.Map;
@@ -31,12 +32,14 @@ public interface AccessorCommandHandler {
     @Accessor
     Set<ICommand> getCommandSet();
 
+    @Mutable
     @Accessor
     void setCommandSet(Set<ICommand> set);
 
     @Accessor
     Map<String, ICommand> getCommandMap();
 
+    @Mutable
     @Accessor
     void setCommandMap(Map<String, ICommand> map);