[turbopack] Don't schedule idle snapshots when we are in a short session (#90804)
In a few `next build` commands i saw multiple snapshots due to this. Idle callbacks were firing near the end of the build.
```
Creating an optimized production build ...
WARNING: The git repository is dirty, but File System Cache is still enabled. Manual removal of the filesystem cache database might be required.
✓ Finished writing to filesystem cache because: idle timeout in 6.1s
✓ Compiled successfully in 69s
✓ Collecting page data using 13 workers in 85ms
Build ran with "compile" mode, to finalize the build run either "generate" or "generate-env" mode as well
✓ Finalizing page optimization in 32ms
Route (app)
...ommitted route table...
✓ Finished writing to filesystem cache because: stop in 3.5s
```
This is a bit wasteful since it is more efficient to write everything in a single batch at the end.
The solution is just to not schedule snapshotting operations in 'short sessions' (aka `next build`)