Fixed plugin env not setting in direnv (#468)

## Summary
cmd.print() was not printing in stdout. Therefore the output was not
being picked up by `eval` in `.envrc`
## How was it tested?
- `devbox init`
- `devbox add nginx`
- `echo $NGINX_CONFDIR`

Author: mohsenari

internal/boxcli/shell.go

@@ -55,8 +55,8 @@ func runShellCmd(cmd *cobra.Command, args []string, flags shellCmdFlags) error {
 		if err != nil {
 			return err
 		}
-		// print to stdout instead of stderr so that direnv can read the output
-		cmd.Print(script)
+		// explicitly print to stdout instead of stderr so that direnv can read the output
+		fmt.Fprint(cmd.OutOrStdout(), script)
 		// return here to prevent opening a devbox shell
 		return nil
 	}