ci: fix bridge-interop cachix skipPush condition never caching on main

[YuanYuYuan]

Summary

The bridge-interop job had a different skipPush condition from every other CI job:

# Wrong — evaluates to true on push events (empty string != repo name)
skipPush: ${{ github.event.pull_request.head.repo.full_name != github.repository }}

# Fixed — consistent with all other jobs
skipPush: ${{ github.event_name == 'pull_request' }}

On a push event, github.event.pull_request is not set, so github.event.pull_request.head.repo.full_name resolves to an empty string. '' != 'ZettaScaleLabs/hiroz' is true, so skipPush was always true — the bridge-interop job never pushed derivations to the hiroz cachix cache, even on main branch runs.

Key Changes

• Fix bridge-interop skipPush to use github.event_name == 'pull_request', matching all other jobs