schroedinger: added mirror

The upstream website and URL are dead presently. This adds a mirror to mitigate the package being unavailable.

The generation of bottles, which this PR will force, should also aid that situation.

Problem reported in [this comment](https://github.com/Homebrew/homebrew/commit/4f4ada9cadc3996077c7fed23c16aa54dea79acd#commitcomment-9071321).