fix(ci): skip tuist auth on fork pull requests
Summary:
- skip `tuist auth login` for fork-originated `pull_request` runs
- keep Tuist OIDC authentication enabled for pushes and same-repo PRs
Rationale:
- fork PR workflows do not get `id-token: write`, so the shared setup
action cannot request a GitHub OIDC token there
- skipping the auth step in that context avoids failing CI before the
actual job work starts
Tests:
- `make check`
- `make build-app`