Mark command computers as onlyOpCanSetNbt

This isn't required in vanilla, as the command computer is a
GameMasterBlock, and so isn't placeable in the first place.

*However*, this is a problem with Create contraptions — with those it's
possible to "place" a command computer complete with NBT. We override
onlyOpCanSetNbt to prevent this [^1].

[^1]: https://github.com/Creators-of-Create/Create/blob/7a7993deb89e432f8a4cce8a8d5f35f85f57bfed/src/main/java/com/simibubi/create/foundation/utility/NBTProcessors.java#L179

projects/common/src/main/java/dan200/computercraft/shared/computer/blocks/AbstractComputerBlockEntity.java

@@ -417,4 +417,9 @@ public Component getCustomName() {
     public Component getDisplayName() {
         return Nameable.super.getDisplayName();
     }
+
+    @Override
+    public boolean onlyOpCanSetNbt() {
+        return getFamily() == ComputerFamily.COMMAND;
+    }
 }