Re-enable gdb stubtest

[srittau]

#15235 did disable stubtest for gdb, due to the following error:

Traceback (most recent call last):
  File "/tmp/stubtest-lhvl4qwb/gdb_stubtest.py", line 22, in <module>
    from mypy.stubtest import main
  File "/tmp/stubtest-lhvl4qwb/lib/python3.13/site-packages/mypy/stubtest.py", line 36, in <module>
    import mypy.build
  File "/tmp/stubtest-lhvl4qwb/lib/python3.13/site-packages/mypy/build.py", line 31, in <module>
    from librt.internal import cache_version
ModuleNotFoundError: No module named 'librt.internal'

This might be related to unsetting LD_LIBRARY_PATH in stubtest_third_party.py. In any case, we should find the error and re-enable stubtest for gdb.