set test=%1
for /f "delims=" %%A in ("!test!") do set "test=%%~A"
echo test after process is "%test%"
rem output: ""ARG=CUSTOM_SOURCE,SOURCE=C:FolderOtherFolderSourceFiles""
:nextVar
   for /F "tokens=1* delims=," %%a in ("%test%") do (
      set %%a
      set test=%%b
   )
if defined test goto nextVar

set test=%ARG%

set test=%test:"=%

set test=%1

:nextVar
   for /F "tokens=1* delims=," %%a in ("%test:"=%") do (
      set %%a
      set test=%%b
   )
if defined test goto nextVar

set test=%ARG%

@set test="ARG=CUSTOM_SOURCE,SOURCE=C:FolderOtherFolderSourceFiles"

@set test=%test:~1,-1%
@echo.%test%