environs/tools: fix SHA256 summing

SHA256 summing of archives was totally
broken; summing the contents rather
than the resulting tgz file. Changed
Archive to just tgz, and added another
function to do the hash.
Merge pull request #577 from axw/environs-tools-archive

environs/tools: fix SHA256 summing

SHA256 summing of archives was totally
broken; summing the contents rather
than the resulting tgz file. Changed
Archive to just tgz, and added another
function to do the hash.