github/actions: Add a workflow to build — heartwood

Since we’re eyeing at a Windows build, we should also have some CI system build on Windows, so that developers that do not run it can access build logs.

Most of the builds currently fail, but that’s okay, the intention is that they become more green over time.

If you are ever spammed by notifications about these on GitHub, disable these at https://github.com/settings/notifications (scroll to “System” and the row “Actions”).

The build results would become available at https://github.com/radicle-dev/heartwood/actions/workflows/build.yml

 name: build
 on:
   push: { branches: [ '*' ] }
   workflow_dispatch:
 concurrency:
   group: ${{ github.sha }}
   cancel-in-progress: true
 jobs:
   build:
     strategy:
       fail-fast: false
       matrix:
         os:
          - macos-latest       # arm64
          - ubuntu-latest-arm  # arm64
          - ubuntu-latest      # x64
          - windows-latest-arm # arm64
          - windows-latest     # x64
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@stable
       - uses: actions/cache@v4
         with:
           path: ~/.cargo
           key: ${{ matrix.os }}-${{ runner.arch }}-${{ hashFiles('Cargo.lock') }}-cargo-home
       - uses: actions/cache@v4
         with:
           path: target
           key: ${{ matrix.os }}-${{ runner.arch }}-${{ hashFiles('Cargo.lock') }}-cargo-target
       - run: cargo build --workspace --verbose --all-features
       - run: cargo test  --workspace --verbose --all-features