using `{{ ref('anything') }}` in a `set_sql_header` call resolves to the current model and not 'anything'

### Describe the bug

Here's a minimal reproducible example (running on BigQuery).

```sql
-- test_tmp_1.sql
{{
config(materialized="table")
}}

select a.* from unnest([
struct(1 as key, 1 as value)
]) as a
```

```sql
-- test_tmp_2.sql
{{
config(materialized="table")
}}

{% call set_sql_header(config) %}
select * from {{ ref('test_tmp_1') }};
{% endcall %}

select * from {{ ref('test_tmp_1') }}
```

When I `dbt run` this project, I get an error when `test_tmp_2` is run saying something along the lines of "test_tmp_2 does not exist".

The output SQL for `test_tmp_2` after a `dbt run` shows:
```sql

select * from `project`.`dataset`.`test_tmp_2`;

create or replace table `project`.`dataset`.`test_tmp_2`


OPTIONS()
as (

select * from `project`.`dataset`.`test_tmp_1`
);

```

### Expected behavior
The `ref('test_tmp_1')` call in the `call set_sql_header(config)` block should resolve to `test_tmp_1`.

Instead it resolves to `test_tmp_2`.

### System information
**Which database are you using dbt with?**
- [ ] postgres
- [ ] redshift
- [x] bigquery
- [ ] snowflake
- [ ] other (specify: ____________)

**The output of `dbt --version`:**
```
installed version: 0.18.0
latest version: 0.18.0

Up to date!

Plugins:
- bigquery: 0.18.0
- snowflake: 0.18.0
- redshift: 0.18.0
- postgres: 0.18.0
```

**The operating system you're using:**

macOS 10.15.6

**The output of `python --version`:**

Python 3.8.1

### Additional context
N/A