make the vm test's priv request wait for the salvage reply

nix/vm-tests/agent-ephemeral-rootfs.nix

@@ -140,8 +140,14 @@ pkgs.testers.runNixOSTest {
             "binds": BINDS,
             "isolation": {"bridge": "${bridge}", "gateway_ip": "${gateway}"},
         })
-        out = host.succeed(f"echo '{req}' | socat - UNIX-CONNECT:/run/hive/priv.sock")
-        assert json.loads(out.strip().splitlines()[-1])["ok"] is True, out
+        # socat gives up 0.5s after its stdin ends unless told to wait; the
+        # salvage copies a whole root, which takes longer.
+        out = host.succeed(f"echo '{req}' | socat -t 120 - UNIX-CONNECT:/run/hive/priv.sock")
+        print("hive-priv answered: " + repr(out))
+        print(host.execute("journalctl -u hive-priv --no-pager | tail -n 30")[1])
+        lines = out.strip().splitlines()
+        assert lines, "hive-priv sent no response"
+        assert json.loads(lines[-1])["ok"] is True, out
 
     def conf():
         return host.succeed(f"cat /etc/nixos-containers/{MACHINE}.conf")