FetchContent: Fix unsetting wrong variable name after provider returns

The unset() command was using __cmake_contentNameLower before that variable was restored from the __cmake_fcCurrentVarsStack. That means if there had been a nested call to FetchContent_MakeAvailable(), the wrong variable name would have been cleared (the nested name instead of the one from the current call). That would have left the variable set upon return, blocking the dependency provider from seeing any further calls to FetchContent_MakeAvailable() in the current variable scope or below for the current dependency.

Backport: release