test/passt.mbuto: Don't fail on missing guest public key

We won't necessarily run mbuto as part of regular tests: it can also
be used for demos or out-of-tree tests.

To keep the profile simple, leave the whole sshd setup there, which
is otherwise harmless, but don't fail if guest-key.pub is missing in
the current directory.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

test/passt.mbuto

@@ -68,7 +68,7 @@ EOF
 	ln -s /run/sshd /usr/share/empty.sshd
 
 	cat > /root/.ssh/authorized_keys <<EOF
-'"$(cat guest-key.pub)"'
+'"$(cat guest-key.pub 2>/dev/null || :)"'
 EOF
 	chmod 600 /root/.ssh/authorized_keys
 	chmod 700 /root