Add Shasums For Released Binaries

[gwynforthewyn]

Expected behavior

When I visit https://github.com/GoogleContainerTools/skaffold/releases, I would like to be able to download or review officially generated sha 256 sums of binaries.

Actual behavior

No such shasums are currently published.

Information

The installation instruction for skaffold is:

curl -Lo skaffold https://storage.googleapis.com/skaffold/releases/v2.13.2/skaffold-linux-amd64 && chmod +x skaffold && sudo mv skaffold /usr/local/bin

For software supply chain security, I'd like to be able to verify that the binary I curl-ed is the same binary that Google intended me to curl. Shasums seem like the normal way of achieving this.

[nathanperkins]

Also, please fix go install since that is a more standard, convenient, and safe way to install go tools: #9327