Fix generate (rtk_lef, lib) scripts with more recent open_pdks/volare builds

[hansemro]

Resolves #1.

This updates some generate scripts to work with newer builds from open_pdks/volare. Didn't encounter an assertion error about sparecell, so I opted to remove it. Please let me know if this is still needed as I plan to support additional corners (https://github.com/hansemro/skywater-130nm/tree/ff_ss_corners_rebuild).

Tested with:

• mflowgen against GcdUnit design from https://code.stanford.edu/ee272/skywater-digital-flow
• volare sky130 build e6f9c8876da77220403014b116761b0b2d79aab4 (2023.02.10)