Build modules: a per-node processor cap, and each step's threads within it (issue 1806)

The operator's desktop slows while it builds (shanks at 92%, the Go build
runtime at 1650% of 24 processors). The build modules declare cpu-cap
(default shanks=50,g14=50,ace=75), which the composer turns into the runtime's
quota at idle priority; each step's go, node and buildah commands get
GOMAXPROCS and UV_THREADPOOL_SIZE for their part of it, and the status tools
show the limits and what the kernel holds the runtime to.

modules/mesh-build-node/cmd/mesh-build-node/fetch_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 	"time"
@@ -245,6 +246,11 @@ func TestNoCommandOfTheStepSeesATokenOfTheProcess(t *testing.T) {
 		if len(c.env) < 3 || !strings.HasPrefix(c.env[0], "PATH=") || !strings.HasPrefix(c.env[1], "HOME=") || c.env[2] != "LC_ALL=C" {
 			t.Errorf("%s was given %v, want PATH, HOME and the locale first", c.name, c.env)
 		}
+		// And its threads, its part of the node's processor cap (issue 1806).
+		if !slices.ContainsFunc(c.env, func(e string) bool { return strings.HasPrefix(e, "GOMAXPROCS=") }) ||
+			!slices.ContainsFunc(c.env, func(e string) bool { return strings.HasPrefix(e, "UV_THREADPOOL_SIZE=") }) {
+			t.Errorf("%s was given %v, without its threads", c.name, c.env)
+		}
 	}
 	// The runner main wires: what it hands a command is exactly what the step gave.
 	ran := forgekey.ExecRunnerClean(10*time.Second)(context.Background(), append(m.baseEnv(), "ONLY=this"), "env")