st/nir: Call st_glsl_to_nir_post_opts before interface unification

Since 95415a54 st_finalize_nir_before_variants calls to nir_shader_gather_info, so finalization should be moved before interface unification.

Closes: #3482 (closed)
Fixes: 95415a54

Passes CI: https://mesa-ci.01.org/global_logic/builds/298/group/63a9f0ea7bb98050796b649e85481845