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/module.go

@@ -202,7 +202,8 @@ func (m *Module) Status(ctx context.Context) map[string]any {
 	err := m.Check(ctx)
 	installed, _ := m.Probe(ctx)
 	r := m.recipe()
-	out := map[string]any{"node": m.Node, "recipe": r.Version(), "installed": installed, "refusing": err != nil}
+	out := map[string]any{"node": m.Node, "recipe": r.Version(), "installed": installed, "refusing": err != nil,
+		"cpu": m.cpuStatus()}
 	if m.Toolchain != nil {
 		out["toolchain-dir"] = m.Toolchain.Paths.Dir(r.Version())
 		out["toolchain-installed"] = m.Toolchain.Present(r.Version())
@@ -227,7 +228,7 @@ const defaultPath = "/usr/local/bin:/usr/bin:/bin"
 // files and git configuration) and a fixed locale. A command is given these and its own words, never this process's
 // environment, which holds the module's bus credentials (ADR 0466 §4).
 func (m *Module) baseEnv() []string {
-	return []string{"PATH=" + pathOrDefault(), "HOME=" + m.Toolchain.homeOrEmpty(), "LC_ALL=C"}
+	return append([]string{"PATH=" + pathOrDefault(), "HOME=" + m.Toolchain.homeOrEmpty(), "LC_ALL=C"}, m.threadEnv()...)
 }
 
 func pathOrDefault() string {