mesa: reset gl_program::info in set_program_string
When called multiple times, the subsequent calls will reuse the original info. This breaks at least st_nir_assign_vs_in_locations because it's a no-op when info.io_lowered is true.
Fixes: 81d106d6 ("radeonsi: lower IO intrinsics - complete rewrite of input/output scanning") Closes: #5001 (closed)