Keep a check build whose key is unchanged instead of building it again

A fresh check asked when only the trunk moved rebuilt every image, though
the reuse key said nothing it is built from changed: a controller check
took about 12 minutes instead of 5.5. The check now keeps a build kept
under its key when a verdict of the build seat names that key and every
digest, names it as the build's own check so a walk takes it, and the
cleanup keeps it while any check names it (novox/hq issue 703).

internal/builder/reuse.go

@@ -494,11 +494,15 @@ func (r Registry) Discard(ctx context.Context, builds []CheckBuild) error {
 func (r Registry) letGo(ctx context.Context, checks, all map[string]checkIndex) (int, error) {
 	store := r.store()
 	stays := map[string]bool{}
+	// recorded are the records under a key another check still names (novox/hq issue 703): a later check that kept
+	// the build names it as the check's that built it, and the walk finds it by that record.
+	recorded := map[string]bool{}
 	for tag, c := range all {
 		if _, going := checks[tag]; going {
 			continue
 		}
 		for _, b := range c.Builds {
+			recorded[b.Module+" "+b.Key+" "+b.Check] = true
 			for _, m := range b.Made {
 				stays[catalogue.Recorded(m.Reference)] = true
 			}
@@ -523,8 +527,10 @@ func (r Registry) letGo(ctx context.Context, checks, all map[string]checkIndex)
 				}
 			}
 			// The record under the key, only while it is still this check's: a later check of the same key
-			// replaced it, and that one is its own check's to let go of.
-			if kept, found, err := r.Find(ctx, b.Module, b.Key); err == nil && found && kept.Check == c.Check {
+			// replaced it, and that one is its own check's to let go of. A build a check kept is that build's
+			// check's record, and goes with the last check naming it (novox/hq issue 703).
+			if kept, found, err := r.Find(ctx, b.Module, b.Key); err == nil && found &&
+				(kept.Check == c.Check || kept.Check == b.Check) && !recorded[b.Module+" "+b.Key+" "+kept.Check] {
 				if err := store.DeleteTagged(ctx, ScratchPrefix+b.Module, tagOf(b.Key)); err != nil && !errors.Is(err, artifacts.Gone) {
 					ok = false
 					failed = append(failed, fmt.Sprintf("%s's record: %v", b.Module, err))