Revert "Allow user specified cache directory (#12)" (#111)

This reverts commit 2d512c5e47.

pkg/utils/utils.go

@@ -12,24 +12,15 @@ import (
 	"golang.org/x/xerrors"
 )
 
-var cacheDir string
-
 func CacheDir() string {
-	if cacheDir == "" {
-		var err error
-		cacheDir, err = os.UserCacheDir()
-		if err != nil {
-			cacheDir = os.TempDir()
-		}
+	cacheDir, err := os.UserCacheDir()
+	if err != nil {
+		cacheDir = os.TempDir()
 	}
 	dir := filepath.Join(cacheDir, "trivy")
 	return dir
 }
 
-func SetCacheDir(cd string) {
-	cacheDir = cd
-}
-
 func FileWalk(root string, targetFiles map[string]struct{}, walkFn func(r io.Reader, path string) error) error {
 	err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
 		if info.IsDir() {