fish: start_ssh_agent: Ignore interactive check with nspawn

We use systemd-nspawn with '--pty', which should allow us to give any
input if necessary but this is necessary to connect a new systemd-run
command with the ssh-agent running inside the guest.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

fish/functions/start_ssh_agent.fish

@@ -14,6 +14,9 @@ function start_ssh_agent -d "Launch an ssh agent only if it has not already been
                 set force true
         end
     end
+    if in_nspawn
+        set force true
+    end
     if not set -q force
         status is-interactive; or return 0
     end