Don't build all wheels all the time

Building wheels, specially on macOS and aarch64, takes a long time.
Instead of building all wheels all the time, let's instead build a
subset of wheels during normal pushes, and all wheels only when we push
a tag.

Signed-off-by: Rodrigo Tobar <rtobar@icrar.org>

.github/workflows/deploy-to-pypi.yml

@@ -4,16 +4,28 @@ name: Build distributions and upload to PyPI
 on: [push, pull_request]
 
 jobs:
+
+  calculate_wheels_to_build:
+    name: Calculate OS/archs to build wheels on
+    runs-on: ubuntu-20.04
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+      macos_archs: ${{ startsWith(github.event.ref, 'refs/tags/v') && 'x86_64 arm64 universal2' || 'auto' }}
+    steps:
+      - name: Calculate strategy matrix
+        id: set-matrix
+        env:
+          MATRIX_INCLUDES: ${{ startsWith(github.event.ref, 'refs/tags/v') && '[{\"os\": \"ubuntu-20.04\", \"arch_linux\": \"aarch64\"}]' || '[]' }}
+        run: echo "matrix={\"os\": [\"ubuntu-20.04\", \"windows-2019\", \"macos-12\"], \"arch_linux\": [\"auto\"], \"include\": $MATRIX_INCLUDES}" >> "$GITHUB_OUTPUT"
+
   build_wheels:
     name: Build wheels on ${{matrix.arch}} for ${{ matrix.os }}
+    needs: calculate_wheels_to_build
     runs-on: ${{ matrix.os }}
     strategy:
-      matrix:
-        os: [ubuntu-20.04, windows-2019, macos-12]
-        arch: [auto]
-        include:
-          - os: ubuntu-20.04
-            arch: aarch64
+      matrix: ${{ fromJson(needs.calculate_wheels_to_build.outputs.matrix) }}
+    env:
+      ARCHS_MACOS: ${{ needs.calculate_wheels_to_build.outputs.macos_archs }}
 
     steps:
       - uses: actions/checkout@v3
@@ -37,8 +49,8 @@ jobs:
         run: |
           python -m cibuildwheel --output-dir wheelhouse
         env:
-          CIBW_ARCHS_LINUX: ${{matrix.arch}}
-          CIBW_ARCHS_MACOS: "x86_64 arm64 universal2"
+          CIBW_ARCHS_LINUX: ${{ matrix.arch_linux }}
+          CIBW_ARCHS_MACOS: ${{ env.ARCHS_MACOS }}
           CIBW_BEFORE_ALL: "bash -c 'cd \"{project}\"; sh .github/tools/install_yajl.sh'"
           CIBW_BUILD_VERBOSITY: 1
           CIBW_ENVIRONMENT_MACOS: "IJSON_EMBED_YAJL=1"