lef: Fixing VNB/VPB properties in .magic.lef files.

Both the `PIN VNB` and `PIN VPB` are now marked with `DIRECTION INOUT`.
The `PIN VNB` is marked with `USE GROUND` and the `PIN VPB` is marked
with `USE POWER`.

This should fix the `.magic.lef` file usage with OpenROAD (and hopefully other
tools too).

Fixes https://github.com/google/skywater-pdk/issues/172

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>