fix: incorrect canonicalUrl set when using output: export (#85019)
After the changes in
https://github.com/vercel/next.js/commit/5ccc9078e094215e6c891c38a31fb94cda03456e,
when in `output: 'export'` mode, the `canonicalUrl` value was getting
corrupted to be set to one of the URLs of the RSC responses (eg
`/another.txt`). This is because the original `url` gets mutated to
append a `.txt` extension for the RSC requests.
This ensures we preserve the original canonical URL so it doesn't get
corrupted when we mutate the pathname.
Failing test ref:
https://github.com/vercel/next.js/actions/runs/18607156298/job/53060147186#step:34:151