Replace go get usage

[ycombinator]

As explained by @fearful-symmetry in #110 (comment):

the go get operation we're using breaks on 1.22:

mage prepare:all                                                                                                                                                
go: modules disabled by GO111MODULE=off; see 'go help modules'
Error: running "go get github.com/elastic/go-licenser" failed with exit code 1

I'm guessing the original code did this since it was a bit of a hack to install binaries without using go install However, this > doesn't work in go 1.22 anymore:
https://tip.golang.org/doc/go1.22

go get is no longer supported outside of a module in the legacy GOPATH mode (that is, with GO111MODULE=off). Other build commands, such as go build and go test, will continue to work indefinitely for legacy GOPATH programs.

[elasticmachine]

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