`CREATE EXTENSION citus` resets `search_path`

Also applies to 10.2 and probably to older versions.

I also tested with another extension, pg_stat_statements, and have seen that it doesn't change the `search_path`, so it looked to me a bug (?):

```sql
[local] onurctirtir@onurctirtir:10700-19616=# create schema my_schema;
CREATE SCHEMA
Time: 4.187 ms
[local] onurctirtir@onurctirtir:10700-19616=# set search_path to my_schema ;
SET
Time: 1.228 ms
[local] onurctirtir@onurctirtir:10700-19616=# create extension citus ;
CREATE EXTENSION
Time: 419.548 ms
[local] onurctirtir@onurctirtir:10700-19616=# show search_path ;
┌─────────────────┐
│ search_path │
├─────────────────┤
│ "$user", public │
└─────────────────┘
(1 row)

Time: 1.207 ms
```