qa: set locale to C.UTF-8 in tox.ini

qa: set locale to C.UTF-8 in tox.ini

as ansible is using UTF-8 encoded characters in the file names, so,
to avoid failures like:

  File "/home/jenkins-build/build/workspace/ceph-pull-requests/qa/.tox/py3/lib/python3.10/site-packages/pip/_internal/utils/unpacking.py", line 217, in untar_file
    with open(path, "wb") as destfp:
UnicodeEncodeError: 'latin-1' codec can't encode characters in position 137-140: ordinal not in range(256)

we have to set a locale which is able to handle UTF-8.

see also https://github.com/ceph/teuthology/pull/1671

Signed-off-by: Kefu Chai <tchaikov@gmail.com>