ci: Install deps & build SoC in CI

[gatecat]

This only sets up the tools and run Amaranth -> GDS; it doesn't do DRC or any other checks on the resulting GDS.

The way the tools are set up could perhaps be improved longer term; for now it's cribbed from how nextpnr does it which has worked well enough in practice.

[gatecat]

I was initially trying with actions/setup-python but I couldn't get it to work with pyqt5 (as pip pyqt5 requires too new a Qt version, so system pyqt5 and hence system python is easier)

[robtaylor]

How to use actions/checkout@2 for external repos:

This part here is cloning a second repository

# While cloning the repository:
#   - it clones the repo into the given `path`
#   - it checks out the branch defined at `ref`
- name: Clone supercharge docs repository
  uses: actions/checkout@v2
  with:
    repository: supercharge/docs
    path: resources/docs/2.x
    ref: 2.x