_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.

buildstream/_cas/cascache.py

@@ -22,7 +22,6 @@ import itertools
 import os
 import stat
 import errno
-import tempfile
 import uuid
 import contextlib
 
@@ -130,7 +129,7 @@ class CASCache():
             else:
                 return dest
 
-        with tempfile.TemporaryDirectory(prefix='tmp', dir=self.tmpdir) as tmpdir:
+        with utils._tempdir(prefix='tmp', dir=self.tmpdir) as tmpdir:
             checkoutdir = os.path.join(tmpdir, ref)
             self._checkout(checkoutdir, tree)
 
@@ -375,7 +374,7 @@ class CASCache():
                     for chunk in iter(lambda: tmp.read(4096), b""):
                         h.update(chunk)
                 else:
-                    tmp = stack.enter_context(tempfile.NamedTemporaryFile(dir=self.tmpdir))
+                    tmp = stack.enter_context(utils._tempnamedfile(dir=self.tmpdir))
                     # Set mode bits to 0644
                     os.chmod(tmp.name, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)
 
@@ -843,7 +842,7 @@ class CASCache():
             # already in local repository
             return objpath
 
-        with tempfile.NamedTemporaryFile(dir=self.tmpdir) as f:
+        with utils._tempnamedfile(dir=self.tmpdir) as f:
             remote._fetch_blob(digest, f)
 
             added_digest = self.add_object(path=f.name, link_directly=True)
@@ -853,7 +852,7 @@ class CASCache():
 
     def _batch_download_complete(self, batch):
         for digest, data in batch.send():
-            with tempfile.NamedTemporaryFile(dir=self.tmpdir) as f:
+            with utils._tempnamedfile(dir=self.tmpdir) as f:
                 f.write(data)
                 f.flush()
 
@@ -950,7 +949,7 @@ class CASCache():
 
     def _fetch_tree(self, remote, digest):
         # download but do not store the Tree object
-        with tempfile.NamedTemporaryFile(dir=self.tmpdir) as out:
+        with utils._tempnamedfile(dir=self.tmpdir) as out:
             remote._fetch_blob(digest, out)
 
             tree = remote_execution_pb2.Tree()