Fix tosa-reference model upstream tracking (#6156)

Summary:
It appears that upstream updates to branch tosa_reference_model/v0.8 which we used to track broke ET CI. Hard coding it to a known good version for now.

* Failed CI job - https://github.com/pytorch/executorch/actions/runs/11283289096/job/31382443033#step:12:26700

* Upstream branch updates - https://review.mlplatform.org/q/project:tosa%252Freference_model+branch:v0.80 Potentially culprit gerrit ids - 12676, 12677, 12678 updated today

Pull Request resolved: https://github.com/pytorch/executorch/pull/6156

Reviewed By: larryliu0820

Differential Revision: D64222142

Pulled By: digantdesai

fbshipit-source-id: 8438770851f8309020755992036f8e4c50857301