The indestructible variable does not work as expected

### Testmerges

#3088,#3271,#3284,#3303

### Description of the bug


This variable seems not to work as expected, at least on structures and objects like it. Tried it on a crate and it was consistently destroyed by explosions of any type I threw at it. I strongly suspect some funny instance where this is only processed for specific object types or something, but I was not able to determine the cause myself.

### What's the difference with what should have happened?

As it says in the description of the variable, I would expect that an item with indestructible set to 1 does not get destroyed by explosions. Currently it gets reduced to sheets of metal.

### How do we reproduce this bug?

1.Place a crate.
2.Plant C4 on a nearby wall, throw a grenade, use the drop bomb command, drop an ob, blow yourself up, blow someone else up, the possibilities are endless.
3.See entropy in action.

### Issue Bingo

- [X] Issue could be reproduced at least once
- [X] Issue happened in a recent (less than 7 days ago) round
- [X] Couldn't find an existing issue about this (https://github.com/cmss13-devs/cmss13/issues)