Update nix cabal version & flake inputs

[carbolymer]

Changelog

- description: |
    Update nix cabal version & flake inputs
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
   - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Changes:

• pin haskell.nix flake input to a tag - you can do nix flake update without worrying about haskell.nix upgrade biting you
• update cabal to 3.14 to avoid building its version from source
• update default ghc cross compilation to 9.6.6

This should also limit the number of GHC versions we're pulling into development shell. This had a side effect of breaking GHA due to running out of disk space: https://github.com/IntersectMBO/cardano-cli/actions/runs/12915728375/job/36018250466?pr=1024

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[Jimbo4350]

LGTM