scons: add setup hook

The scons build system is python-based and has a binary named scons. Unlike CMake, it cannot generate makefiles so we end up having to override the build, install, and check phases. I have added the setupHook to the scons package so that integration requires no unique steps - just putting scons in nativeBuildInputs should be enough. sconsFlags controls the flags specifically passed to scons while buildFlags, installFlags, and checkFlags should still be usable. Some packages use different names for the prefix flag. In those cases you will have to set "prefixKey" to something like "PREFIX=" as there are multiple names for the "prefix" used in scons.