test_defer_excchain_dump_ipython was setting fresh env anew, thus not
propagating $LD_PRELOAD that trun sets when libgolang was built with
thread or address sanitizer. This way the subprocess spawned for IPython
was failing with e.g.

        ==152924==ASan runtime does not come first in initial library list; you should either link runtime to your application or manually preload it with LD_PRELOAD.

-> Fix it by only adjusting environment for spawned IPython instead of
setting it completely anew.

Fixes: 09629367 (golang: tests: Add tests for IPython and Pytest
integration patches)