Can't install or upgrade govc due to url change

The govmomi project changed the URL format for release assets, which has in turn broken the govc formula.

The link to the latest release is: https://github.com/vmware/govmomi/releases/download/v0.26.0/govc_Darwin_x86_64.tar.gz

The formula is currently trying to download https://github.com/vmware/govmomi/releases/download/v0.26.0/govc_darwin_amd64.tar.gz, which doesn't exist.