Fix mktemp options for PEM file creation

Incompatible options used for `mktemp` call after shipping `mktemp`
from `coreutils-gplv2` package. Fix options for temporary PEM file
creation.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

update-manager-common.sh

@@ -248,7 +248,7 @@ ED25519SECRET=
 gen_ed25519_keys ()
 {
   # Generate private key in PEM format
-  pemfile="$(mktemp --tmpdir ed25519_XXXXXX.pem)"
+  pemfile="$(mktemp /tmp/ed25519_pem_XXXXXX)"
   openssl genpkey -algorithm ed25519 -outform PEM -out "${pemfile}"
 
   # Based on: http://openssl.6102.n7.nabble.com/ed25519-key-generation-td73907.html