ci: cram: stop ssh from consuming script input

Cram supplies each generated test script to the shell through standard
input. The shared SSH command inherits that input and consumes the
remaining script, leaving the shell at EOF inside a quoted command.

Commit 5fd42aa540 ("ci: cram: use legacy SCP protocol for remote
copies") rewrote CRAM_REMOTE_COMMAND while adding legacy SCP support and
dropped the -n option added for PCF-2222. Job 15640024463 then failed
130-periodicfileupload.t with:

/bin/sh: 152: Syntax error: Unterminated quoted string (no-eol)

Job 15640024462 leaked this salt marker in the same test:

CRAM1 (no-eol)

Restore -n so SSH reads from /dev/null instead of the generated test
script.

Assisted-by: codex:gpt-5.6-sol
Assisted-by: claude:opus-5:1m-context
Signed-off-by: Petr Štetiar <petr.stetiar@prplfoundation.org>
Fixes: 5fd42aa540 ("ci: cram: use legacy SCP protocol for remote copies")
References: PCF-2222

.gitlab/testbed.yml

@@ -25,7 +25,7 @@
     DUT_OBUSPA_VERBOSE_LEVEL: ''
     DUT_OBUSPA_PROTOCOL_TRACE: ''
     TEST_SETUP_TEARDOWN_DURATION: 360
-    CRAM_REMOTE_COMMAND: ssh root@$TARGET_LAN_IP
+    CRAM_REMOTE_COMMAND: ssh -n root@$TARGET_LAN_IP
     CRAM_REMOTE_COPY: scp -O -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null
     CRAM_TEST_SUITE: |
       .gitlab/tests/cram/generic/acceleration-plan-components