aco: Fix format string used when raising validation errors

aco: Fix format string used when raising validation errors

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: c2b1978aa47c ("aco: rework the way various compilation/validation errors are reported")
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7442>