bsp: raspberry-pico: fix SDK sub-build invocation

bsp/raspberry-pico/RP2040/SConstruct

@@ -1,4 +1,5 @@
 import os
+import subprocess
 import sys
 import rtconfig
 
@@ -32,11 +33,17 @@ objs = PrepareBuilding(env, RTT_ROOT)
 ocwd = os.getcwd()
 os.chdir(find_package_path(os.getcwd(), 'raspberrypi-pico-sdk-latest'))
 print("Enter " + os.getcwd())
+package_env = os.environ.copy()
+package_env['RTT_EXEC_PATH'] = rtconfig.EXEC_PATH
+package_command = [sys.executable, '-m', 'SCons']
 if GetOption('clean'):
-    os.system("scons -c")
-else:
-    os.system("scons")
-os.chdir(ocwd)
+    package_command.append('-c')
+try:
+    package_result = subprocess.call(package_command, env=package_env)
+finally:
+    os.chdir(ocwd)
+if package_result:
+    Exit(package_result)
 
 objs.extend(SConscript(os.path.join(os.getcwd(), 'board', 'ports', 'SConscript'), variant_dir='build/board/ports', duplicate=0))