Differentiate between build and release sha256sums.txt

Windows users on our blog and elsewhere are being confused by the fact that the sha256sums for our exes don't match the downloaded files. This problem will probably get worse once we sign the Mac packages..

We might be able to communicate this to them indirectly by providing two sets of sha256sums.txt files: build-sha256sums.txt and release-sha256sums.txt. Or maybe build-sha256sums.txt and sha256sums.txt...