review: apply gemini-code-assist suggestion on _run_kwargs env handling

Use _run_kwargs.get("env", os.environ).copy() + key-mutation instead of
rebuilding env from os.environ directly. Today both forms are equivalent
(no earlier code in _install_package_wheel_first sets _run_kwargs["env"]),
but the .get().copy() pattern survives any future env modification added
upstream of this block without silently throwing it away.

No behavioural change; tests already assert the final HIPCC_COMPILE_FLAGS_APPEND
value, not the env-construction pattern.

Per https://github.com/unslothai/unsloth/pull/5517#discussion_r... (gemini-code-assist[bot])

studio/backend/core/training/worker.py

@@ -259,10 +259,9 @@ def _install_package_wheel_first(
             _gcc_dir = _hipcc_gcc_install_dir()
             if _gcc_dir is not None:
                 _appended = (f"{_existing_flags} --gcc-install-dir={_gcc_dir}").strip()
-                _run_kwargs["env"] = {
-                    **os.environ,
-                    "HIPCC_COMPILE_FLAGS_APPEND": _appended,
-                }
+                _env = _run_kwargs.get("env", os.environ).copy()
+                _env["HIPCC_COMPILE_FLAGS_APPEND"] = _appended
+                _run_kwargs["env"] = _env
                 logger.info(
                     "HIP source build for %s: appended "
                     "--gcc-install-dir=%s to HIPCC_COMPILE_FLAGS_APPEND",