Fix expression value of constant assignment in interpreter

[beta-ziliani]

Fixes #12015
Original code by @asterite in #12015 (comment).

[straight-shoota]

A spec would be nice.

[oprypin]

FYI a nice way to convey authorship is adding this line to the bottom of the commit description:

Co-authored-by: Ary <...>

-this is what GitHub itself adds, there's no other special co-authorship feature that you wouldn't be able to reproduce.
Certainly looks better than appending "by @asterite"

I am excluding the email part from the above example (don't know what it helps to protect from spam, mostly superstition at this point...), so please go to Crystal repo and do git log to search and find out the full line.

[asterite]

I think eventually we could change the behavior of assigning a constant in the REPL (not in interpreted mode) to evaluate the constant's value right away and return the value, but for now this change is fine :-)