Wait out a forge not served for a moment when registering a build (novox/hq issue 486)

A registration asked the forge while node-tools re-served after a tool-runner
rollout, heard "nothing serves gitea.gitea_api", and failed a good build's
plan. The bus's not-served answer is now link.ErrNothingServes (and a timeout
link.ErrNoAnswer), and the registration asks again for up to 90 s on those
alone, never on a refusal; past the bound it fails saying how long it waited.

internal/link/bus_test.go

@@ -3,6 +3,7 @@ package link
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"strings"
 	"testing"
 	"time"
@@ -115,6 +116,15 @@ func TestAskingAToolNobodyServesFailsAtOnce(t *testing.T) {
 	if !strings.Contains(err.Error(), "nothing serves") {
 		t.Errorf("the refusal does not say nobody is there: %v", err)
 	}
+	// And it is ErrNothingServes, through Ask too, so an asker can wait out a tool runner re-serving and
+	// never a refusal (novox/hq issue 486).
+	if !errors.Is(err, ErrNothingServes) {
+		t.Errorf("nothing serving the tool is not ErrNothingServes: %v", err)
+	}
+	if _, err := Ask(context.Background(), bus, "nobody", "home", nil, 30*time.Second); !errors.Is(err, ErrNothingServes) ||
+		!strings.Contains(err.Error(), "nothing serves nobody.home") {
+		t.Errorf("Ask does not say ErrNothingServes in its words: %v", err)
+	}
 }
 
 // And a served tool answers, with no reply queue to declare and no correlation to check.