Always download coredistools package

Always download coredistools package

runtest.sh/cmd currently downloads the coredistools package if COMPlus_GCStress variable is set by TestEnv or test-env options. This is a bit complicated and this doesn't work with a new option for GC stress testing in build/buildtest.cmd. This changeset moves downloading the coredistools package to buildtest.cmd where it takes gcstress option and downloads other packages.

Linux build doesn't have buildtest.sh, so package donwloading is still in
runtest.sh

Commit migrated from https://github.com/dotnet/coreclr/commit/3fa109c73de2b56059f8b7204503efab40d66d86