Install pyinstaller automatically when not available

build.bat

@@ -2,4 +2,8 @@
 cls
 set dirpath=%~dp0
 if "%dirpath:~-1%" == "\" set dirpath=%dirpath:~0,-1%
+if not exist "%dirpath%\env\scripts\pyinstaller.exe" (
+    "%dirpath%\env\scripts\pip" install pyinstaller
+    "%dirpath%\env\scripts\python" "%dirpath%\env\scripts\pywin32_postinstall.py" -install -silent
+)
 "%dirpath%/env/scripts/pyinstaller" build.spec