Cardano Updates

Use the hydra-github-bridge SSE endpoint for real-time build status
detection instead of polling the GitHub API. Reduces wait time from
minutes (poll interval) to seconds (event-driven).

Tested end-to-end in PR #241 — all validation jobs passed.

Use the hydra-github-bridge SSE endpoint for real-time build status
detection instead of polling the GitHub API. Reduces wait time from
minutes (poll interval) to seconds (event-driven).

Tested end-to-end in PR #241 — all validation jobs passed.

Cloudflare now bypasses cache for /status/* paths, enabling real-time
SSE event delivery.

SSE through Cloudflare buffers events. Now the script polls the
bridge's one-shot endpoint every 120s as a safety net alongside
the SSE stream.

When Hydra resolves builds from cache, it creates only aggregate
"required" check-runs but not individual per-build check-runs.
This causes the GHA upload workflow to discover zero -env closures
and skip all container uploads.

Add a fallback path: when no Hydra check-runs ending in "-env"
are found, evaluate the store paths directly from the flake using
nix eval with extra/discover-env.nix. This ensures containers are
always uploaded regardless of Hydra's check-run behavior.

Also install Nix and checkout the repo in the Discover step to
support the fallback evaluation.

Move the iog-libs.nix sync comment from inside the shell heredoc
(where it breaks line continuation and triggers SC2215) to a Nix
comment above the writeShellApplicationWithRuntime block.