hydra-queue-runner: --build-one: correctly handle a cached build

Previously, the build ID would never flow through channels which
exited.

This patch tracks the buildOne state as part of State and exits avoids
waiting forever for new work.

The code around buildOnly is a bit rough, making this a bit weird to
implement but since it is only used for testing the value of improving
it on its own is a bit questionable.

src/hydra-queue-runner/dispatcher.cc

@@ -374,7 +374,6 @@ void State::abortUnsupported()
             if (!build) build = *dependents.begin();
 
             bool stepFinished = false;
-            bool quit = false;
 
             failStep(
                 *conn, step, build->id,
@@ -385,9 +384,9 @@ void State::abortUnsupported()
                     .startTime = now2,
                     .stopTime = now2,
                 },
-                nullptr, stepFinished, quit);
+                nullptr, stepFinished);
 
-            if (quit) exit(1);
+            if (buildOneDone) exit(1);
         }
     }