test: explicitly set TERM=linux for TEST-69-SHUTDOWN

sulogin from the latest util-linux started falling back to vt102 instead
of linux, which makes screen sad (because we install only the linux
terminfo into the test image) and expect trips over the unexpected
warning. Let's just explicitly set TERM=linux before invoking screen to
avoid this.

+ make -C TEST-69-SHUTDOWN setup run
...
INFO:test-shutdown:log in and start screen
root
root
Last login: Sun Mar  3 13:19:31 from 18.191.105.60
-bash-5.2# screen
screen
Cannot find terminfo entry for 'vt102'.
-bash-5.2# ERROR:test-shutdown:Timeout exceeded.

(cherry picked from commit 7a63c5e550d06659096d858dc14dda04726311fa)
(cherry picked from commit 275d720c9d1f2f7a7c563664a4704028a7f41e72)
(cherry picked from commit 58141ed83142b9b2cb2596052295d3b6acf3f212)
(cherry picked from commit ca08e02a4dbdb36f2a4ea6e5f7791820d6be77ca)

test/TEST-69-SHUTDOWN/test.sh

@@ -32,6 +32,7 @@ ExecStart=/bin/true
 EOF
     inst /usr/bin/screen
     echo "PS1='screen\$WINDOW # '" >>"$workspace/root/.bashrc"
+    echo "TERM=linux" >>"$workspace/root/.bash_profile"
     echo 'startup_message off' >"$workspace/etc/screenrc"
     echo 'bell_msg ""' >>"$workspace/etc/screenrc"
 }