component/headless-chromium: set DEPOT_TOOLS_UPDATE to prevent auto-update

see https://chromium.googlesource.com/chromium/tools/depot_tools.git#updating

component/headless-chromium/buildout.cfg

@@ -110,6 +110,7 @@ environment =
   CXX="${:llvm-toolchain}/clang++"
   AR="${:llvm-toolchain}/llvm-ar"
   NM="${:llvm-toolchain}/llvm-nm"
+  DEPOT_TOOLS_UPDATE=0
 
 # Fix permission for group, slapuserXX should be able to access subfolders
 # in headless-chromium location.