Fix path in "Node is dir" error message

[egerlach]

The error message currently displays the path to the source file of the task. This fixes it to be the path of the node being considered.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (7fa5e9a) 97.80% compared to head (30b22aa) 97.80%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #561 +/- ## ======================================= Coverage 97.80% 97.80% ======================================= Files 106 106 Lines 8710 8710 ======================================= Hits 8519 8519 Misses 191 191 ``` | [Flag](https://app.codecov.io/gh/pytask-dev/pytask/pull/561/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytask-dev) | Coverage Δ | | |---|---|---| | [end_to_end](https://app.codecov.io/gh/pytask-dev/pytask/pull/561/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytask-dev) | `82.27% <0.00%> (-0.02%)` | :arrow_down: | | [integration](https://app.codecov.io/gh/pytask-dev/pytask/pull/561/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytask-dev) | `40.49% <0.00%> (ø)` | | | [unit](https://app.codecov.io/gh/pytask-dev/pytask/pull/561/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytask-dev) | `69.69% <100.00%> (ø)` | | 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=pytask-dev#carryforward-flags-in-the-pull-request-comment) to find out more.

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[tobiasraabe]

Thanks a lot, @egerlach!