CI: rename sshd section in testimage.yml

Rename the fragment that installs a SSH daemon to 'sshd' instead of the
generic 'packages'.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/testimage.yml

@@ -9,5 +9,5 @@ local_conf_header:
     TEST_RUNQEMUPARAMS = "slirp"
     TEST_SERVER_IP = "127.0.0.1"
     QEMU_USE_SLIRP = "1"
-  packages: |
+  sshd: |
     IMAGE_FEATURES:append = " ssh-server-dropbear"