hack: add default GOPROXY in Makefile

This is a roundabout workaround for this issue in OLM:
operator-framework/operator-lifecycle-manager#2322

The issue was technically fixed with the v0.19.1 release with the
following PRs:
operator-framework/operator-lifecycle-manager#2331
operator-framework/operator-lifecycle-manager#2336

...however, trying to update the dependency on OLM in ocs-operator
leads to the following issue:
red-hat-storage/ocs-operator#1426

The addition of a default GOPROXY lets us access a cached version of
this package that no longer exists.

Go is wonderful, folks.

Signed-off-by: Jose A. Rivera <[email protected]>

Author: jarrpa

Diff for: hack/make-project-vars.mk

@@ -5,3 +5,4 @@ ENVTEST_ASSETS_DIR := $(PROJECT_DIR)/testbin
 GOBIN ?= $(BIN_DIR)
 GOOS ?= linux
 GOARCH ?= amd64
+GOPROXY ?= https://proxy.golang.org/