account: keep restored ACCOUNT_URL in the EAB-already-bound path

When re-registering an already-bound EAB account (HTTP 400 "not
awaiting external account binding"), the else branch restored
ACCOUNT_URL from ca.conf but the following unconditional
`export ACCOUNT_URL="$_accUri"` clobbered it with an empty _accUri
(never set on that path), so later signed requests failed with
"A Key ID MUST be specified" / "account URL is empty". Assign the
restored value to _accUri so the shared export uses it.
https://github.com/acmesh-official/acme.sh/issues/3382

acme.sh

@@ -4125,7 +4125,7 @@ _regAccount() {
     fi
     _savecaconf "ACCOUNT_URL" "$_accUri"
   else
-    ACCOUNT_URL="$(_readcaconf ACCOUNT_URL)"
+    _accUri="$(_readcaconf ACCOUNT_URL)"
   fi
   export ACCOUNT_URL="$_accUri"