Remove an image's pushed names with its build tag, and take no work under the free-space floor (issue 1834)

The registry push tags the image a second time, so removing only the build tag freed nothing.
The held image a comparison pulls goes after it is read, and a node under the floor leaves the ask
to another build node instead of taking it and failing.

cmd/mesh-builder/busy_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"github.com/novox/mesh-controller/internal/builder"
 	"os"
 	"strings"
 	"sync"
@@ -221,3 +222,18 @@ func TestAnAgentThatCannotMarkABuildTakesNoWorkUntilItCan(t *testing.T) {
 	}
 	released()
 }
+
+// 1834/AC4: given a node under the floor of free space, when the taking loop asks, then it takes nothing, and takes again
+// once there is room.
+func TestANodeUnderTheFloorOfFreeSpaceTakesNoWork(t *testing.T) {
+	was := builder.FreeBytes
+	t.Cleanup(func() { builder.FreeBytes = was })
+	builder.FreeBytes = func(string) (uint64, error) { return 2 << 30, nil }
+	if !noRoom(t.TempDir()) {
+		t.Error("a node with 2 GiB free took work")
+	}
+	builder.FreeBytes = func(string) (uint64, error) { return 200 << 30, nil }
+	if noRoom(t.TempDir()) {
+		t.Error("a node with 200 GiB free refused work")
+	}
+}