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.