ci: bound cores per Nix build

agent-identity: dev3.direct.omp.uyt54www
agent-persona: generalist
agent-supervisor: unavailable
agent-tool: OMP
agent-tool-version: 18.1.7
agent-runtime: OMP 18.1.7
tooling-profile: dotfiles@39a19af

Author: schickling-assistant

.github/workflows/nix.yml

@@ -15,16 +15,17 @@ concurrency:
 jobs:
   check-x86_64-linux:
     runs-on: ubuntu-latest
-    # A healthy full `nix flake check` baseline is ~35m; 90m provides
-    # headroom while still failing boundedly.
+    # `nix flake check` builds many independent Rust derivations. If each Cargo
+    # build sees every runner core, the concurrent derivations oversubscribe the
+    # hosted runner and make the critical path substantially slower.
     timeout-minutes: 90
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/determinate-nix-action@v3
       # `nix flake check` rather than `nix build`: it builds the package *and*
       # evaluates every `checks.*`, so fmt, clippy, the hermetic test suite, and
       # the `--help` smoke test all gate the PR from one entrypoint.
-      - run: nix flake check --print-build-logs
+      - run: nix flake check --cores 1 --print-build-logs
 
   # st2 drives real processes — process groups, signals, pipes, reaping — and those paths
   # are OS behaviour, not portable Rust: #356 and #255 were aarch64-darwin failures no Linux