update rp2040 to use pico-sdk externally

also try to update ci

.github/workflows/build.yml

@@ -62,6 +62,12 @@ jobs:
     - name: Checkout common submodules in lib
       run: git submodule update --init lib/FreeRTOS-Kernel lib/lwip lib/sct_neopixel
 
+    - name: Checkout pico-sdk
+      if: ${{ matrix.family }} == 'rp2040'
+      run: |
+        git clone --depth 1 https://github.com/raspberrypi/pico-sdk ~/pico-sdk
+        echo >> $GITHUB_ENV PICO_SDK_PATH=~/pico-sdk
+
     - name: Set Toolchain URL
       run: echo >> $GITHUB_ENV TOOLCHAIN_URL=https://github.com/xpack-dev-tools/arm-none-eabi-gcc-xpack/releases/download/v10.2.1-1.1/xpack-arm-none-eabi-gcc-10.2.1-1.1-linux-x64.tar.gz