[Pages] fix: `_error` page's `req.url` can be overwritten to dynamic param on minimal mode (#82347)
In minimal mode, if there was `/[slug].js` with `notFound: true` on the
same segment of `_error.js`, `req.url` and `ctx.asPath` were returned as
`/[slug]`, since it was re-initialized to the request url.
Therefore, don't re-initialize the URL if it is in minimal mode && is
the `_error` page.
Fixes NEXT-4660
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Co-authored-by: JJ Kasper <jj@jjsweb.site>