Fix printing install errors redirected to stdout (python-poetry#4466)

Following a regression in python-poetry#4443

install-poetry.py

@@ -440,7 +440,7 @@ def _is_self_upgrade_supported(x):
             self.install(version)
         except subprocess.CalledProcessError as e:
             print(
-                colorize("error", f"\nAn error has occurred: {e}\n{e.stderr.decode()}")
+                colorize("error", f"\nAn error has occurred: {e}\n{e.stdout.decode()}")
             )
 
             return e.returncode