Fix for ROOT-9055. Avoid searching in the standard system environment variables (i.e. PATH)

cmake/modules/RootNewMacros.cmake

@@ -251,7 +251,7 @@ function(ROOT_GENERATE_DICTIONARY dictionary)
       list(APPEND _list_of_header_dependencies ${fp})
     else()
       find_file(headerFile ${fp} HINTS ${localinclude} ${incdirs} NO_DEFAULT_PATH)
-      find_file(headerFile ${fp})
+      find_file(headerFile ${fp} NO_SYSTEM_ENVIRONMENT_PATH)
       if(headerFile)
         list(APPEND headerfiles ${headerFile})
         list(APPEND _list_of_header_dependencies ${headerFile})