Multi corner for `OpenROAD.STAMidPNR`

[kareefardi]

Description

OpenROAD.STAMidPNR steps are usually followed by resizer steps. Resizer steps by default uses multiple corners. Hence, there are a bit of difference in the STA results between OpenROAD.STAMidPNR and resizer steps.

Proposal

Make OpenROAD.STAMidPNR use multiple corners for more consistency and ease of debugging