chore: mention SST_STAGE (#1211)

- we can use `SST_STAGE` to declare the application stage
- it can be declared in your `.env` or as environment variable

cmd/sst/main.go

@@ -226,6 +226,10 @@ var root = &cli.Command{
 					"2. Store this in the `.sst/stage` file and reads from it in the future.",
 					"",
 					"This stored stage is called your **personal stage**.",
+					"Another option is to use `SST_STAGE` environment variable. You can declare it in `.env` or before the CLI.",
+					"```bash frame=\"none\"",
+					"SST_STAGE=dev sst [command]",
+					"```",
 				}, "\n"),
 			},
 		},