Build modules: no thread limit without the composer's share; with it, the share over their own capacity (ADR 0553 round 2)

Change A rebuilds the three build modules, so their runtimes restart once; it
changes nothing else about their builds until change B gives them the share.

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

@@ -226,6 +226,7 @@ func mustReadDir(t *testing.T, dir string) []string {
 // are given PATH, HOME and the locale and their own words; and the real clean runner, run on `env` here, prints only
 // those.
 func TestNoCommandOfTheStepSeesATokenOfTheProcess(t *testing.T) {
+	t.Setenv("MESH_BUILD_CPU_SHARE", "50")
 	t.Setenv("MESH_BUS_CREDENTIAL", "hunter2")
 	t.Setenv("NPM_TOKEN", "hunter3")
 	files := typeScriptModule(`{"dependencies":{"lodash":"^4"}}`)
@@ -246,7 +247,7 @@ 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).
+		// And its threads, its part of the node's share the composer gave (ADR 0553).
 		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)