aco: Fix format string used when raising validation errors

Encountered while writing a unit test that failed validation on an instruction like p_unit_test 0, %3, where the validation error wasn't printed to stderr but instead triggered an assert fail in vasprintf.

Validation errors mention the pretty-printed instruction including operands with the reserved % character, which caused vasprintf to expect more format arguments than aco provided.

Fixes: c2b1978a ("aco: rework the way various compilation/validation errors are reported")