ci: pass secrets to unit tests reusable workflow (#772)

## Summary

Adds `secrets: inherit` to the `unit_tests` job in
`.github/workflows/_tests.yaml` so `CODECOV_TOKEN` is forwarded to the
reusable workflow at
`apify/workflows/.github/workflows/python_unit_tests.yaml@main`.

## Why

The reusable unit-tests workflow declares `CODECOV_TOKEN` as a
`workflow_call` secret and gates its upload step on `if:
env.CODECOV_TOKEN != ''`. Without `secrets: inherit` (or an explicit
`secrets:` map) on the caller, the secret is empty inside the called
workflow, so the upload step is silently skipped on every commit and the
`unit` flag is never sent to Codecov — Codecov shows it as
carried-forward.

This is why PRs (e.g. #771) only see the `integration` flag in the
Codecov comment and patch coverage numbers reflect only integration
tests. `secrets: inherit` on the grandparent (`on_pull_request.yaml`)
does not auto-propagate through an intermediate caller — it must be set
on each `uses:` invocation.

`apify/apify-sdk-python` already has `secrets: inherit` set on the same
reusable workflow call.