Default cache to "packer_cache" in CWD

packer.go

@@ -36,16 +36,18 @@ func main() {
 
 	defer plugin.CleanupClients()
 
-	var cache packer.Cache
-	if cacheDir := os.Getenv("PACKER_CACHE_DIR"); cacheDir != "" {
+	cacheDir := os.Getenv("PACKER_CACHE_DIR")
+	if cacheDir == "" {
+		cacheDir = "packer_cache"
+	}
+
 	if err := os.MkdirAll(cacheDir, 0755); err != nil {
 		fmt.Fprintf(os.Stderr, "Error preparing cache directory: \n\n%s\n", err)
 		os.Exit(1)
 	}
 
 	log.Printf("Setting cache directory: %s", cacheDir)
-		cache = &packer.FileCache{CacheDir: cacheDir}
-	}
+	cache := &packer.FileCache{CacheDir: cacheDir}
 
 	envConfig := packer.DefaultEnvironmentConfig()
 	envConfig.Cache = cache