Add a 'latest' release tag

[zijchen]

sql-action downloads go-sqlcmd from your GitHub releases page, but the download URL is bound to a specific version number so each time there's a new release, we need to update our code manually: Azure/sql-action#141

A 'latest' tag would help us get the latest and greatest while not having to go update the version number each time 😄

[shueybubbles]

@zijchen This URL should work

https://github.com/microsoft/go-sqlcmd/releases/latest

We just need to make sure the publish task sets it as the latest when creating the release, no need to tag the repo itself. @apoorvdeshmukh is fixin up the github publish task and can take this into account. I set it manually for v0.10.0