Migrate Build System to Match OpenLane 2 (#2115)

+ Repository is now a Nix flake
~ Change all invocations of `openroad -python` to use `run_odbpy_script` for consistency
~ Change build system from ad-hoc to Nix, still producing a Docker image as a final result
~ Update KLayout scripts to use `klayout-pymod` or properly parse commandline arguments
~ `open_pdks` -> `bdc9412` to match OpenLane 2
- Remove local installer; `nix run .` will run OpenLane natively