use right arith explanation fn to fix regressions (#5231)

use right arith explanation fn to fix regressions (#5231)

Use the Node-returning variants of
Constraint::externalExplainByAssertions in TheoryArithPrivate locations
that aren't computing proofs.

The problem is that the TrustNode returning variant requires that arith
has stored an externally valid literal for the constraint, which is not
always the case.

This is what we did on proof-new, I just forgot.