Merge pull request #5374 from ryoon/posix-shell-portability

Fix POSIX shell portability

acme.sh

@@ -1442,7 +1442,7 @@ _toPkcs() {
   else
     ${ACME_OPENSSL_BIN:-openssl} pkcs12 -export -out "$_cpfx" -inkey "$_ckey" -in "$_ccert" -certfile "$_cca"
   fi
-  if [ "$?" == "0" ]; then
+  if [ "$?" = "0" ]; then
     _savedomainconf "Le_PFXPassword" "$pfxPassword"
   fi