ci: cap CARGO_BUILD_JOBS at 1 to stop the DinD engine OOM-killing

The recent CI failures ("Cannot connect to the Docker daemon") are the
DinD build engine being OOM-killed mid-compile, not flakiness. Serialize
both the check and test_nvim cargo builds to jobs=1. Temporary mitigation
pending more host RAM.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.dagger/src/hephaestus_ci/main.py

@@ -20,9 +20,10 @@ class HephaestusCi:
             dag.container()
             .from_("rust:1-bookworm")
             .with_exec(["rustup", "component", "add", "clippy", "rustfmt"])
-            # Cap parallel rustc — unbounded (= ncpu) spikes memory on heavy
-            # crates and OOMs the build engine on a many-core runner.
-            .with_env_variable("CARGO_BUILD_JOBS", "4")
+            # Serialize compilation (jobs=1) — even 4 parallel rustc invocations
+            # spike memory on heavy crates and OOM-kill the DinD build engine.
+            # Slower, but the runner survives; revisit if the host gets more RAM.
+            .with_env_variable("CARGO_BUILD_JOBS", "1")
             .with_mounted_cache(
                 "/usr/local/cargo/registry",
                 dag.cache_volume("heph-cargo-registry"),
@@ -83,7 +84,7 @@ class HephaestusCi:
                 ]
             )
             .with_env_variable("PATH", "/opt/nvim/bin:$PATH", expand=True)
-            .with_env_variable("CARGO_BUILD_JOBS", "4")
+            .with_env_variable("CARGO_BUILD_JOBS", "1")
             .with_directory("/workspace", src)
             .with_workdir("/workspace")
             .with_mounted_cache("/workspace/target", dag.cache_volume("heph-target"))