Make aseg-derived mask only once

### A short summary of what you would like to see in CABINET.

This is very minor, but in postbibsnet, 2 brainmasks are derived from the final segmentations (in native T1w and T2w spaces). It would be cleaner to generate a single brainmask from the BIBSNet output segmentation (chirality correction doesn't matter for this) and then just register it to native T1w and T2w spaces in the same manner as we do with the segmentation:
https://github.com/DCAN-Labs/CABINET/blob/t2output/src/utilities.py#L1242

### Do you have any interest in helping implement the feature?

Yes!

### Add any additional information or context about the request here.

_No response_