To make both messages to be printed on stderr

Custom_Hooks/eb_hooks.py

@@ -4,7 +4,7 @@ import os
 from easybuild.tools.run import run_cmd
 from easybuild.tools.config import build_option
 from easybuild.tools.config import install_path
-from easybuild.tools.build_log import EasyBuildError, print_warning
+from easybuild.tools.build_log import EasyBuildError, print_msg, print_warning
 from easybuild.tools.toolchain.toolchain import SYSTEM_TOOLCHAIN_NAME
 from easybuild.toolchains.compiler.systemcompiler import TC_CONSTANT_SYSTEM
 
@@ -77,7 +77,7 @@ def installation_vetoer(ec):
                 f"packages can't be installed in {system_name}:\n"
             )
         for package in VETOED_INSTALLATIONS[system_name]:
-            print(f"- {package}")
+            print_msg(f"- {package}", stderr=True)
         exit(1)
 
 def parse_hook(ec, *args, **kwargs):