WIP: build aarch64 wheels on release branches

.github/workflows/release-artifacts.yml

@@ -94,29 +94,55 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04, macos-10.15]
+        # is_pr is a flag used to exclude certain jobs from the matrix on PRs.
+        # It is not read by the rest of the workflow.
         is_pr:
           - ${{ startsWith(github.ref, 'refs/pull/') }}
+        include:
+          - os: ubuntu-20.04
+            emulate_aarch64: false
+          - os: ubuntu-20.04
+            emulate_aarch64: true
+          - os: macos-10.15
+            emulate_aarch64: false
 
         exclude:
           # Don't build macos wheels on PR CI.
           - is_pr: true
             os: "macos-10.15"
+          # Don't build ARM linux wheels on PR CI either.
+          - is_pr: true
+            emulate_aarch64: true
 
     steps:
+      - name: Debug
+        run: echo '${{ toJSON(matrix) }}'
+
       - uses: actions/checkout@v3
 
       - uses: actions/setup-python@v3
 
+      - name: Set up QEMU to emulate arm64
+        if: matrix.emulate_aarch64
+        uses: docker/setup-qemu-action@v2
+        with:
+          platforms: arm64
+
       - name: Install cibuildwheel
         run: python -m pip install cibuildwheel==2.9.0 poetry==1.2.0
 
       # Only build a single wheel in CI.
-      - name: Set env vars.
+      - name: "Env: build single wheel for CI"
         run: |
           echo "CIBW_BUILD="cp37-manylinux_x86_64"" >> $GITHUB_ENV
         if: startsWith(github.ref, 'refs/pull/')
 
+      # Only build a single wheel in CI.
+      - name: "Env: build aarch64 wheels"
+        run: |
+          echo "CIBW_ARCHS_LINUX=aarch64 >> $GITHUB_ENV
+        if: matrix.emulate_aarch64
+
       - name: Build wheels
         run: python -m cibuildwheel --output-dir wheelhouse
         env: