Makefile: use GOPROXY

Using a goproxy is up to ten times faster.  Setting it speeds up go
v1.11.x and v1.12.x.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

Makefile

+export GOPROXY=https://proxy.golang.org
+
 GO ?= go
 EPOCH_TEST_COMMIT ?= 394c06f491fe9f1c28a410e3b0b91916a5119406
 DESTDIR ?=