Fixes (part of) #69330

We were using a `u32` here instead of a `u64` for no good reason.

The workaround if you're encountering this is to just use `u32::MAX`
instead, which should be large enough for all practical purposes:
`4294967295`.

## Test Plan

Build turbopack in the next.js repo:

```
pnpm pack-next
```

Follow the repro in #69330:

```
git clone https://github.com/Enkratia/css-reload-show-2.git
# edit the package.json following the instructions from `pnpm pack-next` above
rm package-lock.json
pnpm i
pnpm dev --turbo
```

## Integration Testing?

There are some tests in
`test/integration/image-optimizer/test/index.test.ts`, but I didn't see
the obviously best way to test this, as most of those tests are looking
for errors by scanning stderr, and this is about not generating any
error.

Either way, I think this is very unlikely to regress.