Add `_` for variables inside workgraph engine

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.39%. Comparing base (5937b88) to head (8f09dce). Report is 29 commits behind head on main.

Files	Patch %	Lines
aiida_workgraph/engine/workgraph.py	92.75%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #227 +/- ## ========================================== + Coverage 75.75% 79.39% +3.63% ========================================== Files 70 61 -9 Lines 4615 4469 -146 ========================================== + Hits 3496 3548 +52 + Misses 1119 921 -198 ``` | [Flag](https://app.codecov.io/gh/aiidateam/aiida-workgraph/pull/227/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | Coverage Δ | | |---|---|---| | [python-3.11](https://app.codecov.io/gh/aiidateam/aiida-workgraph/pull/227/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | `79.25% <92.85%> (+3.59%)` | :arrow_up: | | [python-3.12](https://app.codecov.io/gh/aiidateam/aiida-workgraph/pull/227/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | `79.27% <92.85%> (?)` | | | [python-3.9](https://app.codecov.io/gh/aiidateam/aiida-workgraph/pull/227/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | `79.32% <92.85%> (+3.58%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.