_cas/cascache.py: Use utils._tempdir() and utils._tempnamedfile()

The direct usage of tempfile.TemporaryDirectory() and
tempfile.NamedTemporaryFile() here causes leakage of any temporary
data when the process operating on temporary data is terminated
with SIGTERM.

Using the utilities ensures that trash is not left behind in
~/.cache/buildstream/artifacts/tmp when the user terminates
BuildStream with ^C.