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

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.