Add make variable to change the version of Go
Add make variable to change the version of Go

This provides a quick way to build or vet our code against different
releases of Go. For example, the `golang.org/dl` module installs Go 1.15
as an executable named `go1.15` and using it requires only one variable:

    make GO=go1.15

CrunchyData/postgres-operator,CrunchyData/postgres-operator