Fix CI config (#12771)

* Fix CI config

- -mod=vendor flag was not being passed during the build stage, forcing
it to download everything again.
- Optimize the git clone of webapp with --depth=1 and use GIT_SSH_COMMAND to clone.
- Add $(GOFLAGS) to some other places which got missed out.

* Remove GOFLAGS from go list command

It does not work for Go 1.12. Need to fix later.

Makefile

@@ -78,7 +78,7 @@ TESTFLAGS ?= -short
 TESTFLAGSEE ?= -short
 
 # Packages lists
-TE_PACKAGES=$(shell go list ./...)
+TE_PACKAGES=$(shell $(GO) list ./...)
 
 # Plugins Packages
 PLUGIN_PACKAGES=mattermost-plugin-zoom-v1.1.1
@@ -104,7 +104,7 @@ else
 	IGNORE:=$(shell rm -f imports/imports.go)
 endif
 
-EE_PACKAGES=$(shell go list ./enterprise/...)
+EE_PACKAGES=$(shell $(GO) list ./enterprise/...)
 
 ifeq ($(BUILD_ENTERPRISE_READY),true)
 ALL_PACKAGES=$(TE_PACKAGES) $(EE_PACKAGES)
@@ -154,7 +154,7 @@ govet: ## Runs govet against all packages.
 	env GO111MODULE=off $(GO) get golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
 	$(GO) vet $(GOFLAGS) $(ALL_PACKAGES) || exit 1
 	$(GO) vet -vettool=$(GOPATH)/bin/shadow $(GOFLAGS) $(ALL_PACKAGES) || exit 1
-	$(GO) run plugin/checker/main.go
+	$(GO) run $(GOFLAGS) plugin/checker/main.go
 
 gofmt: ## Runs gofmt against all packages.
 	@echo Running GOFMT