[metalos][oss] more buck stuff

Summary:
X-link: facebookincubator/metalos#8

Fix location of `toolchains/`
Copy antlir's `test_target_graph.bxl`

Test Plan:
Export to PR
```
❯ arc skycastle run //tools/skycastle/workflows2/metalos/oss/metalos-oss-tests.sky:test_target_graph
```
https://www.internalfb.com/sandcastle/workflow/3643412098545995848

Differential Revision: D58952290

fbshipit-source-id: b6e9f69e50e24d648407792614b4a54e13c7592f

antlir/antlir2/features/rpm/plan.bzl

@@ -55,7 +55,7 @@ def plan(
         rootless: bool,
         items: Artifact | typing.Any,
         label: Label,
-        flavor: FlavorInfo,
+        flavor: FlavorInfo | typing.Any,
         build_appliance: BuildApplianceInfo | typing.Any,
         parent_layer_contents: LayerContents | None,
         dnf_available_repos: list[Dependency],