Update release metadata for 9.6.1
Account for the changes described in https://github.com/haskell/ghcup-metadata/pull/72:
- Prefer to use the Ubuntu 20.04 and 18.04 binary distributions on Ubuntu and Linux Mint.
- Prefer to use the Rocky 8 binary distribution on unknown distributions.
- Add the released testsuite tarball to the generated ghcup metadata.