package/go: add a HOST_GO_HOST_ENV variable

package/go: add a HOST_GO_HOST_ENV variable

package/go/go.mk provides a HOST_GO_TARGET_ENV which provides a useful
set of environment variables needed to build target Go packages.

For host packages, we simply have package/pkg-golang.mk defining
GO_HOST_ENV to specify CFLAGS/LDFLAGS, but that's it: we don't pass an
explicit path to the compiler, we don't pass GO111MODULE, GOCACHE,
GOROOT, etc.

This commit introduces a HOST_GO_HOST_ENV variable that provides the
appropriate set of environment variables to use when building host
golang packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>