Fix eden du --clean on MacOS

Summary:
`eden du --clean` currently fails with

```
Fatal Python error: initfsencoding: unable to load the file system codec
ModuleNotFoundError: No module named 'encodings'
```
Full error: P149352812

It looks like this is because Buck expects to run with a different python, so
here I clear out the PYTHONHOME variable before we start buck.

This reuses out logic used elsewhere to clean up the environment before
calling buck.

Reviewed By: wez

Differential Revision: D24904105

fbshipit-source-id: 73587c52aff3ea00f68339eb44e7042329de2e44

eden/fs/cli/buck.py

@@ -41,9 +41,9 @@ def is_buckd_running_for_path(path: str) -> bool:
     return proc_utils.new().is_process_alive(buckd_pid)
 
 
-def stop_buckd_for_path(path: str) -> None:
-    print(f"Stopping buck in {path}...")
-
+# Buck is sensitive to many environment variables, so we need to set them up
+# properly before calling into buck
+def run_buck_command(buck_command: List[str], path: str) -> subprocess.CompletedProcess:
     # Using BUCKVERSION=last here to avoid triggering a download of a new
     # version of buck just to kill off buck.  This is specific to Facebook's
     # deployment of buck, and has no impact on the behavior of the opensource
@@ -69,8 +69,8 @@ def stop_buckd_for_path(path: str) -> None:
         elif k.startswith("FB_PAR") or k.startswith("PYTHON"):
             del env[k]
 
-    subprocess.run(
-        ["buck", "kill"],
+    return subprocess.run(
+        buck_command,
         stdout=subprocess.PIPE,
         stderr=subprocess.PIPE,
         cwd=path,
@@ -79,6 +79,12 @@ def stop_buckd_for_path(path: str) -> None:
     )
 
 
+def stop_buckd_for_path(path: str) -> None:
+    print(f"Stopping buck in {path}...")
+
+    run_buck_command(["buck", "kill"], path)
+
+
 def stop_buckd_for_repo(path: str) -> None:
     """Stop the major buckd instances that are likely to be running for path"""
     for project in find_buck_projects_in_repo(path):

eden/fs/cli/main.py

@@ -21,6 +21,7 @@
 from typing import Any, Dict, List, Optional, Set, Tuple, Type
 
 import thrift.transport
+from eden.fs.cli.buck import run_buck_command
 from eden.fs.cli.telemetry import TelemetrySample
 from eden.fs.cli.util import check_health_using_lockfile, wait_for_instance_healthy
 from eden.thrift.legacy import EdenClient, EdenNotRunningError
@@ -402,7 +403,8 @@ def usage_for_redirections(
                     redirection_repos.add(redir_full_path)
                     if clean:
                         print(f"\nReclaiming space from redirection: {redir_full_path}")
-                        subprocess.check_call(["buck", "clean"], cwd=parent)
+                        result = run_buck_command(["buck", "clean"], parent)
+                        result.check_returncode()
                         print("Space reclaimed.\n")
 
         # Deal with any legacy bind mounts that may have been made