fix(nextjs): Adjust path to requestAsynStorageShim.js template file

[Lms24]

Looks like we missed adding the build dir to the template path of requestAsyncStorageShim in the NextJS SDK when we changed the package-internal directory structure a while ago (IIRC this was pre-v8 but not sure actually)

I'm a bit surprised that this works at all, given we use exports conditions and this is a path to a specific file not covered by any exports entry. But I guess webpack can handle this still.

EDIT: Well turns out, this doesn't work. We'll need to add the exports entries.

fixes #13925

closes #13926
closes #13927

[lforst]

This code path is hit when we cannot find the actual request async storage module inside the Next.js package.

We can merge this as-is but the actual fix hides somewhere in #13920

[Lms24]

@lforst thx for checking. Just FYI: This fallback still won't work because of the path not being declared in exports. Canary tests still failed.

[lforst]

@Lms24 ah thanks, I'll get that fixed.