Digtron compat improvement

[thomasrudin]

https://forum.minetest.net/viewtopic.php?p=339460#p339460

[SwissalpS]

These item-names begin with 'digtron' and do not have group:digtron

• digtron:loaded_crate
• digtron:empty_locked_crate
• digtron:empty_crate
• digtron:digtron_core
• digtron:duplicator
• digtron:loaded_locked_crate
  None of them move with a digtron, so the proposed method would work imo.

Edit: the line in question is:

vacuum/dignode.lua

Line 4 in 3aad64e

if oldnode.name and string.sub(oldnode.name, 0, 7) == "digtron" then