Incrementally update ghcup metadata in ghc/ghcup-metadata

This job paves the way for distributing nightly builds

* A new repo https://gitlab.haskell.org/ghc/ghcup-metadata stores the
  metadata on the "updates" branch.
* Each night this metadata is downloaded and the nightly builds are
  appended to the end of the metadata.
* The update job only runs on the scheduled nightly pipeline, not just
  when NIGHTLY=1.

Things which are not done yet

* Modify the retention policy for nightly jobs
* Think about building release flavour compilers to distribute nightly.

Fixes #23334