CI: haskell-actions/setup@v2 resolves to 3.9.3 which cannot be installed (GHCup-00130) #8619

Problem

The CI workflow uses haskell-actions/setup@v2 with enable-stack: true and stack-version: latest. The latest resolves to stack 3.9.3, which GHCup cannot install on the runner:

[ Error ] [GHCup-00130] The version "3.9.3" of the tool stack is not installed.

Impact

This blocks ALL new PRs from first-time contributors and cache-miss builds. It does NOT affect master because master has a GHCup cache from before the metadata issue.

Root Cause

The runner has GHCup binary version 0.2.6.2, which uses a metadata file that lists stack 3.9.3 as available. However, the actual binary for 3.9.3 is not downloadable (stackage metadata/CI infrastructure mismatch).

Workaround Used

In our PR (#8611), we worked around this by:

1. Setting enable-stack: false in the build job (use system stack)
2. Removing haskell-actions/setup@v2 from test jobs entirely (they don't need GHC/stack)

Proposed Fix

Either:

• Pin stack-version to a known-working version (e.g., 3.7.1 or 3.11.1)
• Or upgrade the runner's GHCup binary to the latest version
• Or remove haskell-actions/setup@v2 and use pre-installed system stack