[make-fuchsia-vol] fix the zircon-build-dir env var

The environment variable is now called ZIRCON_BUILDROOT, not
ZIRCON_BUILD_DIR.

Change-Id: I8905af494dc7c0717189aaa1fbc589d677b83f92
diff --git a/garnet/go/src/make-fuchsia-vol/make-fuchsia-vol.go b/garnet/go/src/make-fuchsia-vol/make-fuchsia-vol.go
index 9621c1f..b306bbb 100644
--- a/garnet/go/src/make-fuchsia-vol/make-fuchsia-vol.go
+++ b/garnet/go/src/make-fuchsia-vol/make-fuchsia-vol.go
@@ -30,7 +30,7 @@
 
 var (
 	fuchsiaBuildDir = flag.String("fuchsia-build-dir", os.Getenv("FUCHSIA_BUILD_DIR"), "fuchsia build dir")
-	zirconBuildDir  = flag.String("zircon-build-dir", os.Getenv("ZIRCON_BUILD_DIR"), "zircon build dir")
+	zirconBuildDir  = flag.String("zircon-build-dir", os.Getenv("ZIRCON_BUILDROOT"), "zircon build dir")
 	zirconToolsDir  = flag.String("zircon-tools-dir", os.Getenv("ZIRCON_TOOLS_DIR"), "zircon tools dir")
 
 	bootloader = flag.String("bootloader", "", "path to bootx64.efi")