MM-6839: search relative to executable (#8853)

* MM-6839: searching for paths relative to executable

In addition to searching relative to the current working directory, also
search relative to the location of the binary. This helps locate config
and i18n files when invoking an absolute path to the mattermost binary.

* MM-6839: find mattermost/ binary using utils.FindFile

cmd/platform/main.go

@@ -6,19 +6,10 @@ package main
 import (
 	"fmt"
 	"os"
-	"path/filepath"
 	"syscall"
-)
 
-func findMattermostBinary() string {
-	for _, file := range []string{"./mattermost", "../mattermost", "./bin/mattermost"} {
-		path, _ := filepath.Abs(file)
-		if stat, err := os.Stat(path); err == nil && !stat.IsDir() {
-			return path
-		}
-	}
-	return "./mattermost"
-}
+	"github.com/mattermost/mattermost-server/utils"
+)
 
 func main() {
 	// Print angry message to use mattermost command directly
@@ -33,7 +24,14 @@ The platform binary will be removed in a future version.
 	args := os.Args
 	args[0] = "mattermost"
 	args = append(args, "--platform")
-	if err := syscall.Exec(findMattermostBinary(), args, nil); err != nil {
+
+	realMattermost := utils.FindFile("mattermost")
+	if realMattermost == "" {
+		// This will still fail, of course.
+		realMattermost = "./mattermost"
+	}
+
+	if err := syscall.Exec(utils.FindFile("mattermost"), args, nil); err != nil {
 		fmt.Println("Could not start Mattermost, use the mattermost command directly.")
 	}
 }