go get: installing executables with 'go get' in module mode is deprecated

[vit1251]

Use section compile from source in docs/BUILDING.md execute go get receive next output:

$ go get github.com/github/git-sizer
go: downloading github.com/github/git-sizer v1.5.0
go: downloading github.com/spf13/pflag v1.0.5
go: downloading github.com/cli/safeexec v1.0.0
go: downloading golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
go get: installing executables with 'go get' in module mode is deprecated.
	Use 'go install pkg@version' instead.
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

Use go version 1.17.1