[CORE] Update script sourcing to print stderr to console

The function `source_script` that runs scripts in source mode, has the
method `subprocess.run` that now:
- redirects the stdout to python in order to process the output
- regularly print to terminal stderr (it can be expoited in post-scripts to
print to the user).

Signed-off-by: Nicola Sparnacci <nicola.sparnacci@seco.com>

src/seco_setup_utils.py

@@ -105,10 +105,12 @@ def source_script(script_path, *args, cwd=None):
     args_str = ' '.join(args)
     command = f"bash -c 'source {script_path} {args_str} > /dev/null; env'"
 
-    # Run the command and capture the environment variables
+    # Run the command and capture the environment variables. stdout is 
+    # redirected to python so that it can be processed and it's not 
+    # printed. stderr, instead, is printed to the terminal.
     result = subprocess.run(command, shell=True, 
                             stdout=subprocess.PIPE, 
-                            stderr=subprocess.PIPE, 
+                            stderr=None, 
                             universal_newlines=True,
                             cwd=cwd)