Honor PIP_INDEX_URL for V8 Python dependencies (#61255)

## What changed

Configure V8's `pip.parse` in `MODULE.bazel` to expand `PIP_INDEX_URL` and pass it as `--index-url`, defaulting to `https://pypi.org/simple` when unset or empty. Keep `--require-hashes` enabled.

GitOrigin-RevId: b049056e98bf525906a727d8932178f0a0528cd2

MODULE.bazel

@@ -880,7 +880,11 @@ engine_repository(
 
 # All V8 inputs are owned here, including the pinned Python generator inputs.
 pip.parse(
-    extra_pip_args = ["--require-hashes"],
+    envsubst = ["PIP_INDEX_URL"],
+    extra_pip_args = [
+        "--require-hashes",
+        "--index-url=${PIP_INDEX_URL:-https://pypi.org/simple}",
+    ],
     hub_name = "v8_python_deps",
     python_version = "3.11",
     requirements_lock = "@v8//:bazel/requirements.txt",