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<https://github.com/The-OpenROAD-Project/OpenLane/pull/1046|#1046 Pdn update>
Pull request opened by <https://github.com/maliberty|maliberty>
```
Update pdn_cfg.tcl to use only commands supported by the new pdngen

The prior code was a mix of new commands and old tcl vars that are no
longer supported by the new pdngen.  I find the old setup confusing
so this is my best guess at a correct replacement.

```
<https://github.com/The-OpenROAD-Project/OpenLane|The-OpenROAD-Project/OpenLane>
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