cli: Add "LEGO_PATH" environment variable (#1045)

Co-authored-by: Ludovic Fernandez <ldez@users.noreply.github.com>

cmd/flags.go

@@ -50,9 +50,10 @@ func CreateFlags(defaultPath string) []cli.Flag {
 			Usage: "(deprecated) Filename of the generated certificate.",
 		},
 		cli.StringFlag{
-			Name:  "path",
-			Usage: "Directory to use for storing the data.",
-			Value: defaultPath,
+			Name:   "path",
+			EnvVar: "LEGO_PATH",
+			Usage:  "Directory to use for storing the data.",
+			Value:  defaultPath,
 		},
 		cli.BoolFlag{
 			Name:  "http",

cmd/lego/main.go

@@ -29,11 +29,12 @@ func main() {
 		fmt.Printf("lego version %s %s/%s\n", c.App.Version, runtime.GOOS, runtime.GOARCH)
 	}
 
-	defaultPath := ""
+	var defaultPath string
 	cwd, err := os.Getwd()
 	if err == nil {
 		defaultPath = filepath.Join(cwd, ".lego")
 	}
+
 	app.Flags = cmd.CreateFlags(defaultPath)
 
 	app.Before = cmd.Before