tox: Tell it that our trun is ok to run

trun is not installed by setuptools and so is present only in original
working tree from where tox is run itself. And recent version of tox
(hit with 4.14.2) started to complain about it with

    py312-thread: commands[0] /home/kirr/src/tools/go/pygolang-master/.tox/py312-thread/lib/python3.12/site-packages> /home/kirr/src/tools/go/pygolang-master/trun /home/kirr/src/tools/go/pygolang-master/.tox/py312-thread/bin/python -m pytest gpython/ golang/
    py312-thread: failed with /home/kirr/src/tools/go/pygolang-master/trun (resolves to /home/kirr/src/tools/go/pygolang-master/trun) is not allowed, use allowlist_externals to allow it
      py312-thread: FAIL code 1 (142.22 seconds)

-> Tell tox that our trun is ok to run to fix that.

/reviewed-by @jerome
/reviewed-on nexedi/pygolang!24

tox.ini

@@ -76,3 +76,5 @@ commands=
 # likewise for python debug builds.
         asan,tsan,py{27,39,310,311,312}d: -s    \
         gpython/ golang/
+
+allowlist_externals={toxinidir}/trun