package/go: explicitly set GOMODCACHE

go mod vendor caches downloaded modules to the Go module cache, which
defaults to $GOPATH/pkg/mod - But can be overridden with the GOMODCACHE
environment variable:

https://go.dev/ref/mod#module-cache

So explicitly set GOMODCACHE= for reproducibility.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 162f656884)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/go/go.mk

@@ -26,6 +26,7 @@ HOST_GO_COMMON_ENV = \
 	GOROOT="$(HOST_GO_ROOT)" \
 	GOPATH="$(HOST_GO_GOPATH)" \
 	GOCACHE="$(HOST_GO_TARGET_CACHE)" \
+	GOMODCACHE="$(HOST_GO_GOPATH)/pkg/mod" \
 	GOPROXY=off \
 	PATH=$(BR_PATH) \
 	GOBIN= \