Check if go exists before generating the GOPATH (#10100)

* Check if go exists before generating the GOPATH

* Move export PATH into Go protected section

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

Makefile

 GO ?= go
 SED_INPLACE := sed -i
 SHASUM ?= shasum -a 256
-GOPATH ?= $(shell $(GO) env GOPATH)
+HAS_GO = $(shell hash $(GO) > /dev/null 2>&1 && echo "GO" || echo "NOGO" )
+
+ifeq ($(HAS_GO), GO)
+	GOPATH ?= $(shell $(GO) env GOPATH)
+	export PATH := $(GOPATH)/bin:$(PATH)
+endif
 
-export PATH := $(GOPATH)/bin:$(PATH)
 
 ifeq ($(OS), Windows_NT)
 	EXECUTABLE ?= gitea.exe