flake.lock: Update

Flake lock file updates:

• Updated input 'haskell-nix':
    'github:input-output-hk/haskell.nix/131f8cc' (2026-05-16)
  → 'github:input-output-hk/haskell.nix/04de970' (2026-06-20)
• Updated input 'haskell-nix/hackage':
    'github:input-output-hk/hackage.nix/a482aef' (2026-05-16)
  → 'github:input-output-hk/hackage.nix/9141d76' (2026-06-20)
• Updated input 'haskell-nix/hackage-for-stackage':
    'github:input-output-hk/hackage.nix/ab71329' (2026-05-16)
  → 'github:input-output-hk/hackage.nix/d509fdb' (2026-06-20)
• Updated input 'haskell-nix/iserv-proxy':
    'github:stable-haskell/iserv-proxy/3f7b281' (2026-04-08)
  → 'github:stable-haskell/iserv-proxy/8cdc446' (2026-05-10)
• Updated input 'haskell-nix/stackage':
    'github:input-output-hk/stackage.nix/c1f217f' (2026-05-16)
  → 'github:input-output-hk/stackage.nix/58b68ed' (2026-06-20)

Author: github-actions[bot]