Cmd use GOMODCACHE env (#733)

* use cache path

cmd/kratos/internal/base/mod.go

@@ -4,7 +4,6 @@ import (
 	"bufio"
 	"bytes"
 	"io/ioutil"
-	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"
@@ -46,10 +45,17 @@ func ModuleVersion(path string) (string, error) {
 
 // KratosMod returns kratos mod.
 func KratosMod() string {
-	gopath := os.Getenv("GOPATH")
+	// go 1.15+ read from env GOMODCACHE
+	cacheOut, _ := exec.Command("go", "env", "GOMODCACHE").Output()
+	cachePath := strings.Trim(string(cacheOut), "\n")
+	pathOut, _ := exec.Command("go", "env", "GOPATH").Output()
+	gopath := strings.Trim(string(pathOut), "\n")
+	if cachePath == "" {
+		cachePath = filepath.Join(gopath, "pkg", "mod")
+	}
 	if path, err := ModuleVersion("github.com/go-kratos/kratos/v2"); err == nil {
 		// $GOPATH/pkg/mod/github.com/go-kratos/kratos@v2
-		return filepath.Join(gopath, "pkg", "mod", path)
+		return filepath.Join(cachePath, path)
 	}
 	// $GOPATH/src/github.com/go-kratos/kratos
 	return filepath.Join(gopath, "src", "github.com", "go-kratos", "kratos")