fix: use go version specified by package maintainer
the BUILD_GO_VERSION variable is used only to overrule the debian/control go version. It is used in cases where the golang-any or similar package is specified which defaults to the dists golang.
for example it defaults to golang 1.19, but a package maintainer wants 1.20.
Updating the alternatives late makes sure the setting is not overwritten by a dependency install from the .dsc