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/isolate.go

@@ -35,7 +35,7 @@ func (m *Module) isolation(root string, write bool) buildstep.Isolation {
 		ReadOnly:    []string{m.toolchainDir()},
 		Caches:      []string{m.npmCache()},
 		WriteCaches: write,
-		Env:         []string{"PATH=" + pathOrDefault(), "LC_ALL=C"},
+		Env:         append([]string{"PATH=" + pathOrDefault(), "LC_ALL=C"}, m.threadEnv()...),
 	}
 }