Makefile: unset GOOS for get-envoy command (#1088)

Ingress Controller can be built on macOS by running a command like

  GOOS=linux make build && docker build . -t ingress-controller:dev

except that this will attempt to build and run the get-envoy command for
Linux as well, which won't work. Let's update the Makefile 'envoy'
target to unset the GOOS variable so that the get-envoy command is
always built for the host OS.

Makefile

@@ -110,7 +110,7 @@ envoy-ci: envoy
 .PHONY: envoy
 envoy:
 	@echo "==> $@"
-	mkdir -p ./pomerium/envoy/bin && cd ./pomerium/envoy/bin && go run github.com/pomerium/pomerium/pkg/envoy/get-envoy
+	mkdir -p ./pomerium/envoy/bin && cd ./pomerium/envoy/bin && env -u GOOS go run github.com/pomerium/pomerium/pkg/envoy/get-envoy
 
 UI_DIR = $(shell go list -f {{.Dir}} github.com/pomerium/pomerium/ui)
 internal/ui: