package/go: set GOCACHE for download post-process

The go mod vendor call in support/download/go-post-process accesses the go
cache, so pass GOCACHE= in the environment to ensure our cache directory is
used.

The go cache defaults to ~/.cache/go-build if not set, so this fixes builds
where that location (or GOCACHE if set in the environment) is not writable:

rm -rf ~/.cache/go-build
chmod -w ~/.cache
make docker-compose-source
..
failed to initialize build cache at /home/peko/.cache/go-build: mkdir /home/peko/.cache/go-build: permission denied
make[1]: *** [package/pkg-generic.mk:189: /home/peko/source/buildroot/output/build/docker-compose-2.14.0/.stamp_downloaded] Error 1

We use two different cache directories for target and host builds, but the
download/vendoring should be independent of the architecture, so use the
target variant even for host-only packages for simplicity.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>