Fix dangling promise in unstable-cache (#79248)
While investigating an issue with intermittent `unstable_cache` calls
failing to set noticed we have these dangling promises which cause
"racey" behavior with setting to a cache in environments where the
function stops after the request is finished.
These have been present since
https://github.com/vercel/next.js/commit/b305c2d309b4d4923c3b11f7e258077729ab28fb
altho since racey may or may not have been noticed.
x-ref: [slack
thread](https://vercel.slack.com/archives/C02K2HCH5V4/p1747322956802819?thread_ts=1746633220.634939&cid=C02K2HCH5V4)
