buildctl v0.32 has no "cacheonly" exporter, so every push-triggered
validation build since this workflow was added failed with 'exporter
"cacheonly" could not be found' (run 16, 2026-09-08) — unnoticed, since
nothing depended on it. A build with no --output still builds every
stage on both architectures. The post-publish toolchain assertion now
also requires chromium-browser and psql (12.11.1-ci7).