lang/go/go-package: Honor MAKE_ENV and set GOCACHE under WRKDIR

Previously GOCACHE was not passed as environment variable and hence
during the build of packages the cache was stored in `${GO} env
GOCACHE' (${XDG_CACHE_HOME} if defined or ${HOME}/.cache).

Pass GOCACHE so that all cached file will ends up under ${WRKDIR}.

While here also pass both GOPATH and GOCACHE via MAKE_ENV.

Thanks to Antonio Huete Jimenez for spotting this problem!

Index: go-package.mk
===================================================================
RCS file: /cvsroot/pkgsrc/lang/go/go-package.mk,v
retrieving revision 1.14
diff -u -p -r1.14 go-package.mk
--- go-package.mk	8 Jul 2018 13:53:42 -0000	1.14
+++ go-package.mk	28 Aug 2018 17:46:57 -0000
@@ -70,6 +70,9 @@ PRINT_PLIST_AWK+=	/${GO_PLATFORM}/ { gsu
 			"$${GO_PLATFORM}"); \
 			print; next; }
 
+MAKE_ENV+=	GOPATH=${WRKDIR}:${BUILDLINK_DIR}/gopkg 
+MAKE_ENV+=	GOCACHE=${WRKDIR}/.cache/go-build
+
 .if !target(post-extract)
 post-extract:
 	${RUN} ${MKDIR} ${WRKSRC}
@@ -79,12 +82,12 @@ post-extract:
 
 .if !target(do-build)
 do-build:
-	${RUN} env GOPATH=${WRKDIR}:${BUILDLINK_DIR}/gopkg ${GO} install -v ${GO_BUILD_PATTERN}
+	${RUN} ${PKGSRC_SETENV} ${MAKE_ENV} ${GO} install -v ${GO_BUILD_PATTERN}
 .endif
 
 .if !target(do-test)
 do-test:
-	${RUN} env GOPATH=${WRKDIR}:${BUILDLINK_DIR}/gopkg ${GO} test -v ${GO_BUILD_PATTERN}
+	${RUN} ${PKGSRC_SETENV} ${TEST_ENV} ${MAKE_ENV} ${GO} test -v ${GO_BUILD_PATTERN}
 .endif
 
 .if !target(do-install)