Allow developers to set a custom go version via env variables (#25650)

server/Makefile

@@ -99,8 +99,8 @@ GOFLAGS ?= $(GOFLAGS:)
 # We need to export GOBIN to allow it to be set
 # for processes spawned from the Makefile
 export GOBIN ?= $(PWD)/bin
-GO=go
-DELVE=dlv
+GO ?= go
+DELVE ?= dlv
 LDFLAGS += -X "github.com/mattermost/mattermost/server/public/model.BuildNumber=$(BUILD_NUMBER)"
 LDFLAGS += -X "github.com/mattermost/mattermost/server/public/model.BuildDate=$(BUILD_DATE)"
 LDFLAGS += -X "github.com/mattermost/mattermost/server/public/model.BuildHash=$(BUILD_HASH)"