incorrect result when querying global only system variable without specifying the scope

When a scope of system variable is not specified, Dolt uses SESSION by default, which causes this issue.
Dolt:
```
test/main> SELECT @@GLOBAL.activate_all_roles_on_login;
+--------------------------------------+
| @@GLOBAL.activate_all_roles_on_login |
+--------------------------------------+
| 0 |
+--------------------------------------+
1 row in set (0.00 sec)
test/main> SET @@GLOBAL.activate_all_roles_on_login = true;
test/main> SELECT @@GLOBAL.activate_all_roles_on_login;
+--------------------------------------+
| @@GLOBAL.activate_all_roles_on_login |
+--------------------------------------+
| 1 |
+--------------------------------------+
1 row in set (0.00 sec)

test/main> SELECT @@activate_all_roles_on_login;
+-------------------------------+
| @@activate_all_roles_on_login |
+-------------------------------+
| 0 |
+-------------------------------+
1 row in set (0.00 sec)
```
MySQL:
```
mysql> SELECT @@GLOBAL.activate_all_roles_on_login;
+--------------------------------------+
| @@GLOBAL.activate_all_roles_on_login |
+--------------------------------------+
| 0 |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> SET @@GLOBAL.activate_all_roles_on_login = true;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @@GLOBAL.activate_all_roles_on_login;
+--------------------------------------+
| @@GLOBAL.activate_all_roles_on_login |
+--------------------------------------+
| 1 |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT @@activate_all_roles_on_login;
+-------------------------------+
| @@activate_all_roles_on_login |
+-------------------------------+
| 1 |
+-------------------------------+
1 row in set (0.00 sec)
```