fix: use runfiles strategy to get runfiles root

python/runfiles/runfiles.py

diff --git a/python/runfiles/runfiles.py b/python/runfiles/runfiles.py
index ffa2473..1b3e8eb 100644
--- a/python/runfiles/runfiles.py
+++ b/python/runfiles/runfiles.py
@@ -95,6 +95,9 @@
             raise TypeError()
         self._runfiles_root = path
 
+    def _GetRunfilesDir(self) -> str:
+        return self._runfiles_root
+
     def RlocationChecked(self, path: str) -> str:
         # Use posixpath instead of os.path, because Bazel only creates a runfiles
         # tree on Unix platforms, so `Create()` will only create a directory-based
@@ -118,7 +121,7 @@
 
     def __init__(self, strategy: Union[_ManifestBased, _DirectoryBased]) -> None:
         self._strategy = strategy
-        self._python_runfiles_root = _FindPythonRunfilesRoot()
+        self._python_runfiles_root = self._strategy._GetRunfilesDir()
         self._repo_mapping = _ParseRepoMapping(
             strategy.RlocationChecked("_repo_mapping")
         )
@@ -321,19 +324,6 @@
 # Support legacy imports by defining a private symbol.
 _Runfiles = Runfiles
 
-
-def _FindPythonRunfilesRoot() -> str:
-    """Finds the root of the Python runfiles tree."""
-    root = __file__
-    # Walk up our own runfiles path to the root of the runfiles tree from which
-    # the current file is being run. This path coincides with what the Bazel
-    # Python stub sets up as sys.path[0]. Since that entry can be changed at
-    # runtime, we rederive it here.
-    for _ in range("rules_python/python/runfiles/runfiles.py".count("/") + 1):
-        root = os.path.dirname(root)
-    return root
-
-
 def _ParseRepoMapping(repo_mapping_path: Optional[str]) -> Dict[Tuple[str, str], str]:
     """Parses the repository mapping manifest."""
     # If the repository mapping file can't be found, that is not an error: We

tests/runfiles/runfiles_test.py

diff --git a/tests/runfiles/runfiles_test.py b/tests/runfiles/runfiles_test.py
index 03350f3..c6f454f 100644
--- a/tests/runfiles/runfiles_test.py
+++ b/tests/runfiles/runfiles_test.py
@@ -527,7 +527,7 @@
             expected = ""
         else:
             expected = "rules_python"
-        r = runfiles.Create({"RUNFILES_DIR": "whatever"})
+        r = runfiles.Create({"RUNFILES_DIR": os.environ.get("RUNFILES_DIR")})
         assert r is not None  # mypy doesn't understand the unittest api.
         self.assertEqual(r.CurrentRepository(), expected)