Updates for `-static` Stack bindist suffix

[borsboom]

• Update CI build to produce a linux-x86_64-static bindist. It should be the same as the linux-x86_64 version (which is now also static) but the directory name must use linux-x86_64-static since stack upgrade requires the directory name to match the bindist name.

• Modify stack upgrade to download bindists from https://get.haskellstack.org/upgrade/ (e.g. https://get.haskellstack.org/upgrade/linux-x86_64.tar.gz) instead of directly from the Github release, to make it more future proof.

• Modify stack upgrade so that it doesn't require the downloaded tarball's top-level path component to match the name of the tarball. Just strip off the top-level path component (like to tar's --strip-components 1). This will allow us to set up redirects from get.haskellstack.org/upgrade in the future.