Build a merge's modules of a tier at once, and beside the check's tests where the judge cache allows (hq issue 716)

A check built its images one after another, after merge-check.sh. With a judge cache the
builds run during the tests, out of their reach; a failed check lets go of them.

cmd/mesh-builder/main.go

@@ -26,6 +26,7 @@ import (
 	"os/signal"
 	"sort"
 	"strings"
+	"sync"
 	"syscall"
 	"time"
 
@@ -272,7 +273,12 @@ func answer(ctx context.Context, publisher builder.Publisher, on, workspace stri
 	// role's own events under the build's id (novox/hq ADR 0157) — so whoever asked, and anybody
 	// watching, reads the same lines this container's log holds, live, and after the fact from the
 	// stream. Said first, before anything runs, so a build that hangs is one that visibly started.
+	// One line at a time: a check's builds for its merge say their lines while its tests say theirs (novox/hq
+	// issue 716).
+	var saying sync.Mutex
 	say := func(step, message string) {
+		saying.Lock()
+		defer saying.Unlock()
 		fmt.Fprintf(os.Stderr, "  [%s] %s\n", step, message)
 		work.Say(step, message)
 		if mine != nil && step != "run" && step != "output" {