Add variable to skip updating the server

[zzahkaboom24]

Description

Adds the variable "SKIP_UPDATE", to be used in upcoming pterodactyl/yolks#70.

Has been requested on Discord for Hytale, so I added it.

Checklist for all submissions

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you tested and reviewed your changes with confidence that everything works?
• Did you branch your changes and PR from that branch and not from your master branch?
  • If not, why?:

• You verify that the start command applied does not use a shell script
  • If some script is needed then it is part of a current yolk or a PR to add one
• The egg was exported from the panel