CI: Cache Hackage package index in integration tests

[zlav]

Overview

Cache the Hackage package index (~/.cache/cabal/packages) in integration test workflows. cabal update currently downloads the full index (~500 MB) from scratch on every run, taking ~3.5 minutes. With the cache, subsequent runs do an incremental update in seconds.

Acceptance criteria

cabal update in integration test jobs completes in seconds instead of ~3.5 minutes when the cache is warm.

Testing plan

1. First run populates the cache (still ~3.5 min).
2. Subsequent runs restore the cached index — cabal update does an incremental update (~seconds).
3. Verify all 3 test matrix jobs still pass.

Risks

• The cache path (~/.cache/cabal/packages) assumes XDG layout, which is what the Alpine container uses (/github/home/.cache/cabal/packages). If cabal's config changes the index location, the cache would miss.
• Cache key rotates with the release tag. Between releases, the same cache entry is reused and incrementally updated by cabal update.

Metrics

~3.5 min saved per integration test job (4 jobs total: build + 3 test shards = ~14 min saved per push on warm cache).

Checklist

• I added tests for this PR's change (or explained in the PR description why tests don't make sense).
• If this PR introduced a user-visible change, I added documentation into docs/.
• If this PR added docs, I added links as appropriate to the user manual's ToC in docs/README.ms and gave consideration to how discoverable or not my documentation is.
• If this change is externally visible, I updated Changelog.md. If this PR did not mark a release, I added my changes into an ## Unreleased section at the top.
• If I made changes to .fossa.yml or fossa-deps.{json.yml}, I updated docs/references/files/*.schema.json AND I have updated example files used by fossa init command. You may also need to update these if you have added/removed new dependency type (e.g. pip) or analysis target type (e.g. poetry).
• If I made changes to a subcommand's options, I updated docs/references/subcommands/<subcommand>.md.