CI: Add GH Actions workflow to build on each commit.

.github/workflows/build.yml

@@ -0,0 +1,46 @@
+name: Build MicroPython + module
+
+on:
+  push:
+    branches: '*'
+  pull_request:
+    branches: '*'
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        # macos-13 is x86_64, and macos-14 is arm64
+        os: [ubuntu-22.04, macos-13, macos-14]
+      fail-fast: false
+    runs-on: ${{ matrix.os }}
+    name: build.py ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: true
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+      - uses: carlosperate/arm-none-eabi-gcc-action@v1
+        with:
+          release: 10.3-2021.10
+      - name: Install CMake v3.22 via PyPI
+        run: python -m pip install cmake==3.28.3
+      - name: Check Versions
+        run: |
+          arm-none-eabi-gcc --version
+          cmake --version
+          python --version
+          uname -a
+      - name: Initialise micro:bit MicroPython submodules
+        run: git -C micropython-microbit-v2 submodule update --init
+      - name: Build mpy-cross
+        run: make -C micropython-microbit-v2/lib/micropython/mpy-cross -j2
+      - name: Build MicroPython with C++ module
+        run: make -C micropython-microbit-v2/src USER_C_MODULES=../../.. -j2
+      - name: Upload hex file
+        uses: actions/upload-artifact@v4
+        with:
+          name: MICROBIT-MICROPYTHON-${{ github.sha }}-${{ matrix.os }}.hex
+          path: src/MICROBIT.hex