Suppress warning from systemd user session

Anaconda is starting systemd --user for RDP session. This unfortunately,
will result in message:

```
Failed to adjust memory pressure threshold, ignoring: Device or resource
busy
```

This message is printed directly to terminal and bypassing stdout or
stderr.

Force this message to be logged to the journal instead of terminal.

Suggested-by: Lukas Nykryn
(cherry picked from commit f47ca11)

Resolves: RHEL-67911

pyanaconda/display.py

@@ -77,7 +77,10 @@ def start_user_systemd():
 
     # Start the user instance of systemd. This call will also cause the launch of
     # dbus-broker and start a session bus at XDG_RUNTIME_DIR/bus.
-    childproc = util.startProgram(["/usr/lib/systemd/systemd", "--user"])
+    # Without SYSTEMD_LOG_TARGET variable the systemd is logging directly to terminal
+    # bypassing stdout and stderr
+    childproc = util.startProgram(["/usr/lib/systemd/systemd", "--user"],
+                                  env_add={"SYSTEMD_LOG_TARGET": "journal-or-kmsg"})
     WatchProcesses.watch_process(childproc, "systemd")
 
     # Set up the session bus address. Some services started by Anaconda might call

tests/unit_tests/pyanaconda_tests/test_display.py

@@ -45,7 +45,8 @@ def test_start_user_systemd(self, util_mock, conf_mock, watch_mock):
         start_user_systemd()
 
         util_mock.startProgram.assert_called_once_with(
-            ["/usr/lib/systemd/systemd", "--user"]
+            ["/usr/lib/systemd/systemd", "--user"],
+            env_add={"SYSTEMD_LOG_TARGET": "journal-or-kmsg"}
         )
         watch_mock.watch_process.assert_called_once_with(
             100, "systemd"