fix(i/t/tor.go): show correct command line (#673)

While there, ensure that we print a warning if we cannot find
the correct tor binary.

Work part of https://github.com/ooni/probe/issues/1917.

internal/tunnel/tormobile.go

@@ -5,13 +5,16 @@ package tunnel
 // This file implements our strategy for running tor on mobile.
 
 import (
+	"strings"
+
 	"github.com/cretz/bine/tor"
 	"github.com/ooni/go-libtor"
 )
 
 // getTorStartConf in this configuration uses github.com/ooni/go-libtor.
 func getTorStartConf(config *Config, dataDir string, extraArgs []string) (*tor.StartConf, error) {
-	config.logger().Info("tunnel: tor: using ooni/go-libtor")
+	config.logger().Infof("tunnel: tor: exec: <ooni/go-libtor> %s %s",
+		dataDir, strings.Join(extraArgs, " "))
 	return &tor.StartConf{
 		ProcessCreator: libtor.Creator,
 		DataDir:        dataDir,