CI: test on Linux arm64 instead of macOS, and allow ten minutes

.github/workflows/ci.yml

@@ -39,7 +39,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        # x86-64 and arm64: the reads that take no lock depend on the order in
+        # which memory is written, which arm64 keeps less of, and the compiler
+        # inlines differently on each.
+        os: [ubuntu-latest, ubuntu-24.04-arm]
         # The oldest release that go.mod allows, and the newest.
         go: ["1.26.x", stable]
     steps:
@@ -51,7 +54,10 @@ jobs:
           go-version: ${{ matrix.go }}
           cache-dependency-path: bench/go.sum
       - name: Test
-        run: go test -race -shuffle=on -timeout=5m -coverprofile=coverage.out ./...
+        # With the race detector and coverage, the slowest package takes about
+        # four minutes on a typical runner, and a slow runner takes 1.7 times
+        # as long.
+        run: go test -race -shuffle=on -timeout=10m -coverprofile=coverage.out ./...
       - name: Coverage
         run: go tool cover -func=coverage.out | tail -n 1
       - name: Test the benchmarks