+ 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

Tested on Apple M1 and AWS Graviton.

+ Add support for multiple-architecture building
    + Support for amd64, ppc64le and arm64v8 added
    + amd64 and arm64v8 will be automatically built by the CI

~ Major CI Overhauls
   ~ Added composite actions for the Docker build and setting environment variables
   ~ pdk build is now its own job, and amd64/aarch64 have a job each
   ~ matrix -> design_matrix, designs only tested on amd64 for performance reasons

~ run/build base docker images now pushed to Docker Hub based on a hash of dependency lists (to work with the buildx `docker-container` driver)

- Remove pandas (overkill for what we're using it for)

- Remove `cugr`, `drcu` *for now* (they're x86-64 Linux-only utilities)