nether's lava crust destroys digger heads

[programmerjake]

attempting to use a digtron to dig lava crust will successfully dig the lava crust but then replace the location where the lava crust was with a lava source which often destroys digger heads.

lava crust's after_destruct function which causes the destruction: https://github.com/minetest-mods/nether/blob/01b6f3c56d9a22214b819da7ee8e1e5d25768011/nodes.lua#L757