refactor(cron): copy os.environ before sanitizing for subprocess

Matches the env= callsite convention at the other sanitized
subprocess spawns (cua_backend dict(os.environ), gateway
os.environ.copy()). Functionally equivalent — _sanitize_subprocess_env
never mutates its input — but avoids handing the live mapping to the
helper.

Follow-up to salvaged PR #49207.

cron/scheduler.py

@@ -1035,7 +1035,7 @@ def _run_job_script(script_path: str) -> tuple[bool, str]:
             text=True,
             timeout=script_timeout,
             cwd=str(path.parent),
-            env=_sanitize_subprocess_env(os.environ),
+            env=_sanitize_subprocess_env(os.environ.copy()),
             **popen_kwargs,
         )
         stdout = (result.stdout or "").strip()