Fix POSIX shell portability

POSIX standard says test command has '=" as for checking identical.
'==' is bash dialect.
Replace '==' with '='.

See:
https://pubs.opengroup.org/onlinepubs/009604399/utilities/test.html

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