CI: Run specs in release mode

[straight-shoota]

We should run specs in release mode sometimes.

This is to ensure issues in release mode are detected (see #13412 for an example).

Running specs in release mode on every commit is probably too costly. And it should not be necessary. The compiler doesn't do anything different for release mode, except asking LLVM to do its optimization magic. So any issues there are bugs either in LLVM or our codegen and those are relatively rare to appear out of nowhere.

It should be sufficient to run specs in release mode in regular intervals. Nightly builds might be a good start.

[miry]

I see this is a small task for beginners.
I would like to learn about CI/CD for crystal.
If there is no concerns, I could prepare something during this weekends.
In same time, if someone will do it faster - I don't want be a blocker.

[straight-shoota]

Implementing should be easy, yes.

The more complex part is deciding what we want and how we want to do it.
If you have a concrete proposal (or wan to develop one), please go ahead and share it 👍