mgr/dashboard/api: set a UTF-8 locale when running pip

mgr/dashboard/api: set a UTF-8 locale when running pip

ansible-core started to include files whose filenames are encoded in
non-ascii characters, so we have to use a more capable encoding for the
locale in order to install this package. otherwise we'd have following
error:

Collecting ansible-core<2.12,>=2.11.3
  Using cached ansible-core-2.11.4.tar.gz (6.8 MB)
ERROR: Exception:

Traceback (most recent call last):
  File "/tmp/tmp.fX76ASIrch/venv/lib/python3.8/site-packages/pip/_internal/cli/base_command.py", line 173, in _main
    status = self.run(options, args)
...
  File "/tmp/tmp.fX76ASIrch/venv/lib/python3.8/site-packages/pip/_internal/utils/unpacking.py", line 226, 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)

Signed-off-by: Kefu Chai <kchai@redhat.com>