Update on "Use external_deps for sentencepiece"

as title

Differential Revision: [D59770172](https://our.internmc.facebook.com/intern/diff/D59770172/)

[ghstack-poisoned]

Author: helunwencser

shim/xplat/executorch/build/env_interface.bzl

@@ -42,6 +42,7 @@ _EXTERNAL_DEPS = {
     "prettytable": "//third-party:prettytable",
     "pybind11": "//third-party:pybind11",
     "re2": [],  # TODO(larryliu0820): Add support
+    "sentencepiece": [],
     # Core C++ PyTorch functionality like Tensor and ScalarType.
     "torch-core-cpp": "//third-party:libtorch",
     "torchgen": "//third-party:torchgen",