Use user-specific temp directory if set

This allows non-privileged users to use containerd. This is part of a
larger track of work integrating containerd into Cloudfoundry's garden
with support for rootless.

[#156343575]

Signed-off-by: Claudia Beresford <cberesford@pivotal.io>

cmd/ctr/commands/content/content.go

@@ -506,7 +506,7 @@ var (
 )
 
 func edit(rd io.Reader) (io.ReadCloser, error) {
-	tmp, err := ioutil.TempFile("", "edit-")
+	tmp, err := ioutil.TempFile(os.Getenv("XDG_RUNTIME_DIR"), "edit-")
 	if err != nil {
 		return nil, err
 	}