internal/app/spcontainer: resolve path through dispatcher

This prevents state from os tainting the test data.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/app_test.go

@@ -581,6 +581,8 @@ func (k *stubNixOS) lookupEnv(key string) (string, bool) {
 		return "/run/user/1971", true
 	case "XDG_CONFIG_HOME":
 		return "/home/ophestra/xdg/config", true
+	case "DBUS_SYSTEM_BUS_ADDRESS":
+		return "", false
 	default:
 		panic(fmt.Sprintf("attempted to access unexpected environment variable %q", key))
 	}

internal/app/spcontainer.go

@@ -139,7 +139,12 @@ func (s *spFilesystemOp) toSystem(state *outcomeStateSys) error {
 		varRunNscd,
 	}
 
-	_, systemBusAddr := dbus.Address()
+	// dbus.Address does not go through syscallDispatcher
+	systemBusAddr := dbus.FallbackSystemBusAddress
+	if addr, ok := state.k.lookupEnv(dbus.SystemBusAddress); ok {
+		systemBusAddr = addr
+	}
+
 	if entries, err := dbus.Parse([]byte(systemBusAddr)); err != nil {
 		return &hst.AppError{Step: "parse dbus address", Err: err}
 	} else {