### What?
When using the file system cache with `isrMemoryCacheSize: 0`, time-based revalidation is not working, and the file is constantly updated. I have also added some debug logging to mirror that in the `fetch-cache` handler
Detailed explanation in #58507
### Why?
The cached object's tags are incorrectly accessed, causing the cache to be rewritten every hit. This is catastrophic for a caching system that relies on file modification timestamps. The tags are one level up in the object from where [they are currently being accessed](9ab8828f72/packages/next/src/server/lib/incremental-cache/file-system-cache.ts (L178)).
Below shows a cached fetch representation on disk. When written, the tags reside at `obj.tags` instead of `obj.data.tags`
```json
{
"kind": "FETCH",
"data": {
"headers": {
"connection": "keep-alive",
"content-encoding": "br",
"content-type": "application/json; charset=utf-8",
"date": "Wed, 15 Nov 2023 21:17:42 GMT",
"server": "nginx/1.18.0 (Ubuntu)",
"transfer-encoding": "chunked",
"vary": "Accept-Encoding"
},
"body": "[SNIP]",
"status": 200,
"url": "https://timeapi.io/api/Time/current/zone?timeZone=UTC"
# this is where the current code is trying to pull the tags
# "tags": [ "time-with-fetch" ]
},
"revalidate": 20,
# tags actually live here
"tags": [
"time-with-fetch"
]
}
```
Fixes#58507
df4c2aa8ec