Cache directive doesn't work with incremental PPR

[focux]

Link to the code that reproduces this issue

https://github.com/focux/dynamicio-with-inc-ppr-repro

To Reproduce

1. Start the app
2. Run npm run build

Current vs. Expected behavior

When you try to build the app, it throws this error:

Error:   x Only async functions are allowed to be exported in a "use cache" file.

 | export const experimental_ppr = true;

I would expect the use cache directive to work with incremental PPR. It currently work when ppr: true and not incremental.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:03:15 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 20.11.1
  npm: 10.2.4
  Yarn: 1.22.22
  pnpm: 9.15.0
Relevant Packages:
  next: 15.1.1-canary.2 // Latest available version is detected (15.1.1-canary.2).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.3.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

dynamicIO, Partial Prerendering (PPR)

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

No response