CI: Target the tested architecture, not `-march=native`

From here:

When testing certain vector architectures, we should configure the JIT-compiler to not target -march=native, but whatever ISA we're testing, such that these errors can be found also on a system with a more recent version of AVX.

added infrastructure label

changed title from Follow-up from "Various extensions to the vectorizer" to {+CI: Target the tested architecture, not -march=native+}

marked this issue as related to #120

mentioned in merge request !445 (merged)