Use GOARCH=amd64 when testing on 'Apple Silicon' (#462)

There is no arm64 artifact for our current version of kubebuilder-tools,
which causes test setup to fail on M1 machines.  However, the amd64
artifact offered by kubebuilder seems to work well enough for our
purposes, so we can use that as a workaround until we're able to upgrade
kubebuilder-tools down the road.
diff --git a/Makefile b/Makefile
index 7f503cb..7d81ab4 100644
--- a/Makefile
+++ b/Makefile
@@ -227,8 +227,10 @@
 
 ENVTEST_ASSETS_DIR=$(shell pwd)/testbin
 test: manifests generate ## Run the unit tests
-	mkdir -p ${ENVTEST_ASSETS_DIR}
-	test -f ${ENVTEST_ASSETS_DIR}/setup-envtest.sh || curl -sSLo ${ENVTEST_ASSETS_DIR}/setup-envtest.sh https://raw.githubusercontent.com/kubernetes-sigs/controller-runtime/v0.8.3/hack/setup-envtest.sh
+	# Kubebuilder-tools doesn't have a darwin+arm (i.e. Apple Silicon) distribution but the amd one works fine for our purposes
+	if [[ "${GOOS}" == "darwin" && "${ARCH}" == "arm64" ]]; then export GOARCH=amd64; fi; \
+	mkdir -p ${ENVTEST_ASSETS_DIR}; \
+	test -f ${ENVTEST_ASSETS_DIR}/setup-envtest.sh || curl -sSLo ${ENVTEST_ASSETS_DIR}/setup-envtest.sh https://raw.githubusercontent.com/kubernetes-sigs/controller-runtime/v0.8.3/hack/setup-envtest.sh; \
 	source ${ENVTEST_ASSETS_DIR}/setup-envtest.sh; fetch_envtest_tools $(ENVTEST_ASSETS_DIR); setup_envtest_env $(ENVTEST_ASSETS_DIR); GINKGO_EDITOR_INTEGRATION=true go test ./... -coverprofile cover.out
 
 ##@ Helm