Fix bad naming on release script for Windows (#9135)

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: fertapric

lib/mix/lib/mix/tasks/release.init.ex

@@ -263,7 +263,7 @@ defmodule Mix.Tasks.Release.Init do
     if not defined RELEASE_VM_ARGS (set RELEASE_VM_ARGS=!REL_VSN_DIR!\vm.args)
     if not defined RELEASE_DISTRIBUTION (set RELEASE_DISTRIBUTION=sname)
     if not defined RELEASE_BOOT_SCRIPT (set RELEASE_BOOT_SCRIPT=start)
-    if not defined RELEASE_BOOT_SCRIPT (set RELEASE_BOOT_SCRIPT_CLEAN=start_clean)
+    if not defined RELEASE_BOOT_SCRIPT_CLEAN (set RELEASE_BOOT_SCRIPT_CLEAN=start_clean)
     set RELEASE_SYS_CONFIG=!REL_VSN_DIR!\sys
 
     if "%~1" == "start" (set "REL_EXEC=elixir" && set "REL_EXTRA=--no-halt" && set "REL_GOTO=start")