generate-keystore: fix Permission denied error

MacOS permissions are stupid:
```
 > ls -l /dev/stderr
lr-xr-xr-x  1 root  wheel  0 Jun 20 20:45 /dev/stderr -> fd/2
```

Signed-off-by: Jakub Sokołowski <jakub@status.im>

scripts/generate-keystore.sh

@@ -50,4 +50,4 @@ exec keytool -genkey -v \
     -alias "${KEYSTORE_ALIAS}" \
     -storepass "${KEYSTORE_PASSWORD}" \
     -keypass "${KEYSTORE_KEY_PASSWORD}" \
-    > /dev/stderr
+    >&2