Cardano Updates

- DeterminateSystems/nix-installer-action: v9 → v21
- DeterminateSystems/magic-nix-cache-action: v2 → v13
- cachix/cachix-action: v14 → v16

Fixes CI failures caused by TypeError: Invalid URL in the old
nix-installer-action when fetching from install.determinate.systems.

Fix transient HTTP 502 failures when downloading source tarballs from
GitHub under high parallelism (-j 0).

Two changes:

1. Add --retry 3 --retry-connrefused to curl invocations. Curl retries
   transient HTTP errors (408, 429, 500, 502, 503, 504) with built-in
   exponential backoff (1s, 2s, 4s). The existing actionRetry 5 is kept
   as a second layer for non-HTTP failures.

2. Cap concurrent downloads at 20 using Shake's Resource mechanism.
   This prevents hundreds of simultaneous curl processes from
   overwhelming GitHub when running with -j 0. Non-network tasks
   (patching, sdist creation, index building) still run at full
   parallelism.

Flake lock file updates:

• Updated input 'haskell-nix':
    'github:input-output-hk/haskell.nix/cad2645' (2026-01-03)
  → 'github:input-output-hk/haskell.nix/962394c' (2026-02-07)
• Updated input 'haskell-nix/hackage':
    'github:input-output-hk/hackage.nix/05797dd' (2026-01-03)
  → 'github:input-output-hk/hackage.nix/5e6caa6' (2026-02-07)
• Updated input 'haskell-nix/hackage-for-stackage':
    'github:input-output-hk/hackage.nix/fba3874' (2026-01-03)
  → 'github:input-output-hk/hackage.nix/5fcc92e' (2026-02-07)
• Added input 'haskell-nix/hls-2.12':
    'github:haskell/haskell-language-server/7d983de' (2025-09-24)
• Updated input 'haskell-nix/iserv-proxy':
    'github:stable-haskell/iserv-proxy/1506051' (2025-08-15)
  → 'github:stable-haskell/iserv-proxy/91ef7ff' (2026-02-04)
• Updated input 'haskell-nix/stackage':
    'github:input-output-hk/stackage.nix/8a7333a' (2026-01-03)
  → 'github:input-output-hk/stackage.nix/9d52557' (2026-02-07)