Fix UnboundLocalError in ROCm version detection dpkg/rpm fallback (#6149)

* Fix UnboundLocalError in _detect_rocm_version dpkg/rpm fallback

A leftover local import re inside the amd-smi branch made re function
local for the whole scope. When amd-smi and hipconfig are absent and
dpkg-query or rpm reports rocm-core, the epoch strip at the dpkg/rpm
fallback hit re.sub before any local binding existed and crashed the
installer with UnboundLocalError. Drop the local import (the module
already imports re at top level) and add a regression test covering the
dpkg path without hipconfig.

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

studio/install_python_stack.py

@@ -212,7 +212,6 @@ def _detect_rocm_version() -> tuple[int, int] | None:
                 env = _amd_smi_env(),
             )
             if result.returncode == 0:
-                import re
                 m = re.search(r"ROCm version:\s*(\d+)\.(\d+)", result.stdout)
                 if m:
                     return int(m.group(1)), int(m.group(2))