ci: fix package_core

Author: facchinm

.github/workflows/package_core.yml

@@ -5,10 +5,16 @@ on: [push, pull_request]
 jobs:
   build:
     runs-on: ubuntu-latest
-    container: zephyrprojectrtos/ci:latest
     env:
-      CMAKE_PREFIX_PATH: /opt/toolchains
+      ZEPHYR_SDK_INSTALL_DIR: /opt/zephyr-sdk-0.16.8/
     steps:
+      - name: Install toolchain
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends git cmake wget python3-pip
+          wget https://github.com/zephyrproject-rtos/sdk-ng/releases/download/v0.16.8/zephyr-sdk-0.16.8_linux-x86_64_minimal.tar.xz
+          cd zephyr-sdk-0.16.8 && ./setup.sh -t arm-zephyr-eabi -c
+
       - name: Initialize
         working-directory: ArduinoCore-zephyr
         run: |