pdal_execute_pipeline: Add an env_overrides kwarg

kart/point_cloud/__init__.py

@@ -5,13 +5,20 @@
 from kart.cli_util import tool_environment
 
 
-def pdal_execute_pipeline(pipeline):
+def pdal_execute_pipeline(pipeline, *, env_overrides=None):
+    """
+    Executes the given PDAL pipeline. Should be a list of dicts/strings, each representing a PDAL stage.
+    Returns a list of metadata output from each stage.
+    """
     with tempfile.NamedTemporaryFile() as f_metadata:
         env = tool_environment()
         # Until we incorporate a vendored PDAL into our build, it might be built against a libproj that
         # uses a different proj database than the one we're vendoring.
         # So we carefully don't tell it where to look for the proj database.
         env.pop("PROJ_LIB", None)
+        # NOTE: Kart itself doesn't currently use env_overrides, but don't remove it.
+        # PDAL uses environment variables for various purposes, and this is helpful for `kart ext-run` scripts.
+        env.update(env_overrides or {})
         subprocess.run(
             ["pdal", "pipeline", "--stdin", f"--metadata={f_metadata.name}"],
             check=True,