Benchmark tool doesn't work based on documentation

[benjaminclauss]

• https://etcd.io/docs/v3.4/op-guide/performance/#benchmarks
• https://github.com/etcd-io/etcd/tree/v3.4.16/tools/benchmark

➜  ~ go get go.etcd.io/etcd/tools/benchmark
go: go.mod file not found in current directory or any parent directory.
	'go get' is no longer supported outside a module.
	To build and install a command, use 'go install' with a version,
	like 'go install example.com/cmd@latest'
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.