cargo.build: a persistent build cache shared by all worlds

- The cargo target folder is <cache>/cargo-target, next to Wasmtime's
  cache, which moves to <cache>/wasmtime: Wasmtime's cleanup worker
  deletes unknown entries in its folder, also Cargo's artifacts.
- Builds use the SDK at <cache>/sdk/<content hash>/, so cargo reuses
  the compiled SDK in every world. /plugins/cargo/sdk/ stays for agents.
- Tests run in the release profile and share compiled crates with the
  release build.
- The cargo and cli tests use a persistent cache in CARGO_TARGET_TMPDIR:
  warm, the cargo suite takes 11 s (was 235 s) and the cli suite 6 s
  (was 149 s).
- nix flake check runs ignored stress tests. AGENTS.md: rules for fast
  tests.

cli/tests/cli.rs

@@ -12,9 +12,15 @@ use tokio::{
     process::{Child, Command},
 };
 
+const CACHE: &str = concat!(env!("CARGO_TARGET_TMPDIR"), "/cache");
+
 fn binary() -> Command {
     let mut command = Command::new(env!("CARGO_BIN_EXE_sausage"));
-    command.env_remove("SAUSAGE_HEADLESS").kill_on_drop(true);
+    command
+        .arg("--cache-dir")
+        .arg(CACHE)
+        .env_remove("SAUSAGE_HEADLESS")
+        .kill_on_drop(true);
     command
 }
 fn world() -> (tempfile::TempDir, PathBuf) {
@@ -133,12 +139,15 @@ async fn init_builds_agent() {
         std::fs::read(world.join("plugins/agent/source/Cargo.lock")).unwrap(),
         include_bytes!("../../Cargo.lock")
     );
-    let host = Host::start_with_cache(
-        &world,
-        &Path::new(env!("CARGO_MANIFEST_DIR")).join("../target/core/cli-cache"),
-    )
-    .await
-    .unwrap();
+    assert!(!world.join("state/cargo/target").exists());
+    assert!(
+        Path::new(CACHE)
+            .join("cargo-target/wasm32-wasip2/release")
+            .is_dir()
+    );
+    let host = Host::start_with_cache(&world, Path::new(CACHE))
+        .await
+        .unwrap();
     let doc = host.call("agent.rpc.discover", json!({})).await.unwrap();
     assert_eq!(doc["x-sausage-grace"], 600);
     assert!(