Ensure unstable_cache revalidate option enables ISR (#57390)

Noticed that if you use `unstable_cache` it did not opt-in to ISR even though fetch() does opt-in. This ensures the prerender-manifest holds the revalidate value.

packages/next/src/server/web/spec-extension/unstable-cache.ts

@@ -1,3 +1,4 @@
+import { maybePostpone } from '../../../client/components/maybe-postpone'
 import type {
   StaticGenerationStore,
   StaticGenerationAsyncStorage,
@@ -29,6 +30,23 @@ export function unstable_cache<T extends Callback>(
     const store: undefined | StaticGenerationStore =
       staticGenerationAsyncStorage?.getStore()
 
+    if (store && typeof options.revalidate === 'number') {
+      // Revalidate 0 is a special case, it means opt-out of static generation.
+      if (options.revalidate === 0) {
+        maybePostpone(store, 'revalidate: 0')
+        // Set during dynamic rendering
+        store.revalidate = 0
+        // If revalidate was already set in the store before we should check if the new value is lower, set it to the lowest of the two.
+      } else if (typeof store.revalidate === 'number') {
+        if (store.revalidate > options.revalidate) {
+          store.revalidate = options.revalidate
+        }
+        // All other cases we set the value from the option.
+      } else {
+        store.revalidate = options.revalidate
+      }
+    }
+
     const incrementalCache:
       | import('../../lib/incremental-cache').IncrementalCache
       | undefined =