Amend the virtualenv path in pyvenv.cfg command

[hroncok]

pypa/virtualenv@44b7bd6

Without this, test_clone_contents fails with virtualenv 20.38.0+:

                with open(file_in_clone, 'rb') as f:
                    lines = f.read().decode('utf-8')
>                   assert venv_path not in lines,\
                        'Found source path in cloned file %s' % file_in_clone
E                   AssertionError: Found source path in cloned file /tmp/tmp_lsp66c7/clone_venv/pyvenv.cfg
E                   assert '/tmp/tmp_lsp66c7/srs_venv' not in 'home = /usr/bin\nimplementation = CPython\nversion_info = 3.14.3.final.0\nversion = 3.14.3\nexecutable = /usr/bin/python3.14\ncommand = /.../virtualenv-clone/.tox/py314/bin/python3 -m virtualenv /tmp/tmp_lsp66c7/srs_venv\nvirtualenv = 21.1.0\ninclude-system-site-packages = false\nbase-prefix = /usr\nbase-exec-prefix = /usr\nbase-executable = /usr/bin/python3.14\n'
E
E                     '/tmp/tmp_lsp66c7/srs_venv' is contained here:
E                       home = /usr/bin
E                       implementation = CPython
E                       version_info = 3.14.3.final.0
E                       version = 3.14.3
E                       executable = /usr/bin/python3.14
E                       command = /.../virtualenv-clone/.tox/py314/bin/python3 -m virtualenv /tmp/tmp_lsp66c7/srs_venv
E                     ?                                                                                                   +++++++++++++++++++++++++
E                       virtualenv = 21.1.0
E                       include-system-site-packages = false
E                       base-prefix = /usr
E                       base-exec-prefix = /usr
E                       base-executable = /usr/bin/python3.14

Fixes #83