Fixes for sygus-inst (#8448)

Fixes for sygus-inst (#8448)

Fixes two issues:

Symmetry breaking was disabled after latest refactoring of sygus options. This meant that the terms considered by sygus-inst were often redundant.
The instantiation constant attribute is set on evaluation variables. This avoids model-unsoundness issues with sygus-inst, which are caused by using the evaluation variable (itself) in instantiations.
The latter issue was discovered by trying the second issue from cvc5/cvc5-projects#487 with --sygus-inst, where it incorrectly says "sat" after the fix from #8447.

Fixed #8456. Fixes #8457.