ci: adopt a standard GoReleaser configuration

[johannes-riecken]

I tested this locally with goreleaser release --skip=publish. cosign didn't run without prompts, but I assume that environment variables in the pipeline already handle that?

$ tar -tf bin/gt-darwin-amd64.tar.gz
LICENSE
README.md
gt
$ ls -F1 bin
CHANGELOG.md
checksums.txt
config.yaml
gt-0.4.4.tar.gz
gt-darwin-amd64.tar.gz
gt-darwin-arm64.tar.gz
gt-linux-amd64.tar.gz
gt-linux-arm64.tar.gz
gt-windows-amd64.zip
gt_darwin_amd64_v1/
gt_darwin_arm64/
gt_linux_amd64_v1/
gt_linux_arm64/
gt_windows_amd64_v1/