[antlir2] set name on sendstream anon_target

Summary: This doesn't do anything, but it looks nicer in logs.

Test Plan:
```
❯ buck2 build --show-output fbcode//antlir/antlir2/test_images/package:btrfs-test-3.sendstream.v2
BUILD SUCCEEDED
fbcode//antlir/antlir2/test_images/package:btrfs-test-3.sendstream.v2 buck-out/v2/gen/fbcode/9874f87d0ccb406e/antlir/antlir2/test_images/package/__btrfs-test-3.sendstream.v2__/image.sendstream.v2
```

It doesn't show in the logs above because `name` is not part of the cache key
so it didn't even get rebuilt

Reviewed By: justintrudell

Differential Revision: D50794150

fbshipit-source-id: c93d3d098bb0a98e383b501ec947c0fa4e8bcc3c

antlir/antlir2/bzl/package/sendstream.bzl

@@ -104,6 +104,7 @@ def anon_v1_sendstream(
         attrs["layer"] = layer
     if build_appliance:
         attrs["build_appliance"] = build_appliance
+    attrs["name"] = str(attrs["layer"].label.raw_target()) + ".sendstream"
     return ctx.actions.anon_target(
         _sendstream,
         attrs,