Formatter: Better f-string dummy (#5730)
## Summary
The previous dummy was causing instabilities since it turned a string
into a variable.
E.g.
```python
script_header_dict[
"slurm_partition_line"
] = f"#SBATCH --partition {resources.queue_name}"
```
has an instability as
```python
- script_header_dict["slurm_partition_line"] = (
- NOT_YET_IMPLEMENTED_ExprJoinedStr
- )
+ script_header_dict[
+ "slurm_partition_line"
+ ] = NOT_YET_IMPLEMENTED_ExprJoinedStr
```
## Test Plan
The instability is gone, otherwise it's still a dummy
