ci: split relay runtime from polkadot node binary

[karolk91]

The polkadot binary normally embeds a Rust-compiled runtime at build time. To get short-epoch behavior for tests, the project built polkadot with --features fast-runtime. This coupled the node SHA to the runtime version and its features — every CI run paid a full polkadot recompile just to obtain the short-epoch runtime.

The PR splits the relay runtime into a separately-cached artifact and hands it to the node via a chain-spec at startup. The node binary becomes runtime-agnostic.

Flow

1. .github/env exposes two independent version pins: POLKADOT_NODE_VERSION and RELAY_RUNTIME_VERSION.
2. scripts/get_polkadot_binaries.sh builds each group from one bare clone of polkadot-sdk using per-SHA git worktrees + per-SHA CARGO_TARGET_DIR.
3. The polkadot-node group builds polkadot with SKIP_WASM_BUILD=1 — no embedded runtime.
4. The relay-runtime group builds westend-runtime --features fast-runtime and caches westend_runtime.compact.compressed.wasm.
5. scripts/create_westend_local_spec.sh runs chain-spec-builder against that WASM to emit zombienet/westend-local-spec.json.
6. Zombienet spawns polkadot --chain westend-local-spec.json — the runtime is loaded from the spec at startup.

Independent caches

Each group's cache key is polkadot-binaries-${runner.os}-<group>-<VERSION_VAR>. Bumping RELAY_RUNTIME_VERSION invalidates only the relay-runtime cache; the node cache stays warm.

Paseo support

Paseo zombienet tests now run against a real paseo-local relay built from paseo-network/runtimes, not against the westend relay as before.

• New paseo-relay-runtime group in get_polkadot_binaries.sh source-builds paseo-runtime --features fast-runtime from paseo-network/runtimes and caches paseo_runtime.compact.compressed.wasm. Pinned via PASEO_RELAY_RUNTIME_VERSION in .github/env.
• The pinned commit is ed51b038e2
• scripts/create_paseo_local_spec.sh mirrors the westend script and emits zombienet/paseo-local-spec.json.
• zombienet/bulletin-paseo-local.toml sets chain = "paseo-local" and chain_spec_path = "./zombienet/paseo-local-spec.json".
• justfile zombienet recipes parameterize the relay spec script + path by {{runtime}}; examples/justfile does the same for the integration-test harness (relay + parachain chain-spec paths now branch on the runtime).

Source-build workspace hint

The source-build path now exports WASM_BUILD_WORKSPACE_HINT=<worktree>. Without it, substrate-wasm-builder walks up from OUT_DIR
(under our external CARGO_TARGET_DIR) and lands on the bulletin-chain repo's Cargo.lock, which doesn't contain paseo-runtime
— the build script panics with Failed to find entry for package paseo-runtime. Westend happened to work without the hint because
westend-runtime is reachable as a transitive dep from the bulletin workspace; paseo isn't.

Harness wiring

• SDK tests — RelaychainConfigBuilder uses with_chain_spec_path(...) after the typestate-required with_chain(...). Spec path from RELAY_CHAIN_SPEC_PATH env, default ./zombienet/westend-local-spec.json.
• TOML zombienet — chain_spec_path = "./zombienet/westend-local-spec.json" alongside the existing chain = "westend-local" label.

Both invoke create_westend_local_spec.sh before the test; it's a no-op cache hit when the spec is newer than the WASM.