Reduce downloading the (latest) Wash binary tgz to a directly runnable copy-paste command

[ekinanp]

Based on the instructions in https://puppetlabs.github.io/wash/installing_wash, to download the latest Wash binary, users need to:

• Copy-paste the curl command
• Click a link that takes them to https://github.com/puppetlabs/wash/releases/latest to figure out the value of {WASH_VERSION}
• Substitute {WASH_VERSION} in their copy-pasted curl command
• Run the command

Ideally, users should be able to copy-paste the appropriate curl command and run it. So something like

curl -sfLO https://github.com/puppetlabs/wash/releases/download/latest/wash-latest-x86_64-unknown-linux.tgz

This requires changing the name of the generated .tgz file.