github: hello_world_multiplatform: set the toolchain list

The SDK is growing bigger and bigger, set the toolchain list so we only
fetch the ones needed by the tests we run, save some disk space,
apparently this workflow is getting close.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>

Author: fabiobaltieri

.github/workflows/hello_world_multiplatform.yaml

@@ -62,7 +62,7 @@ jobs:
         uses: zephyrproject-rtos/action-zephyr-setup@f7b70269a8eb01f70c8e710891e4c94972a2f6b4 # v1.0.6
         with:
           app-path: zephyr
-          toolchains: all
+          toolchains: aarch64-zephyr-elf:arc-zephyr-elf:arc64-zephyr-elf:arm-zephyr-eabi:mips-zephyr-elf:riscv64-zephyr-elf:sparc-zephyr-elf:x86_64-zephyr-elf:xtensa-dc233c_zephyr-elf:xtensa-sample_controller32_zephyr-elf
 
       - name: Build firmware
         working-directory: zephyr