Start the puller's capacity watch once Serve has the runtime's channel, since a build module pulls before it serves (issue 1887)

A watch asked before Serve is refused, which would have left every build
module on its settings file again. stdio.Served closes when Serve has the
channel; WatchConfigServed waits on it, and Pull starts the watch beside its
loop.

go/buildstep/pull.go

@@ -70,7 +70,8 @@ type Puller struct {
 	// a restart.
 	Load func(key string) ([]byte, error)
 	// WatchConfig, when set, follows the module's config value `capacity` (CapacitySetting) as mesh-config resolves it
-	// for this node: stdio.WatchConfig in a module, a test's own double.
+	// for this node, and wakes the puller on each change (watchCapacity): WatchConfigServed in a module, a test's own
+	// double. Until it hands a value the puller judges by Capacity.
 	WatchConfig func(key string, handle func(stdio.ProvisionChange) error) error
 
 	mu      sync.Mutex
@@ -228,7 +229,7 @@ func (p *Puller) Pull(ctx context.Context) error {
 	if err := p.restore(); err != nil {
 		return err // pulling on a pause that cannot be read could build on a node paused on purpose
 	}
-	p.watchCapacity()
+	go p.watchCapacity() // WatchConfigServed waits for Serve, which a module starts after its pull
 	wait := p.PullWait
 	if wait <= 0 {
 		wait = DefaultPullWait
@@ -409,6 +410,13 @@ func (p *Puller) watchCapacity() {
 	}
 }
 
+// WatchConfigServed is stdio.WatchConfig once Serve has the runtime's channel (stdio.Served): a build module starts its
+// pull before it serves, and a watch asked before that would be refused.
+func WatchConfigServed(key string, handle func(stdio.ProvisionChange) error) error {
+	<-stdio.Served()
+	return stdio.WatchConfig(key, handle)
+}
+
 // capacityFrom is the node's capacity in one value the watch handed, and why it is 0 when it cannot be read.
 func (p *Puller) capacityFrom(c stdio.ProvisionChange) (int, string) {
 	if c.Refused != "" {