fix: fix an edge case that VUE_CLI_SERVICE_CONFIG_PATH might be ignored (#5598)

Though, it is still a bad practice to use `VUE_CLI_SERVICE_CONFIG_PATH`
in a project with `vue.config.js`

closes #5584

Author: haoqunjiang

packages/@vue/cli-service/lib/Service.js

@@ -314,6 +314,7 @@ module.exports = class Service {
       const resolvedPath = p && path.resolve(this.context, p)
       if (resolvedPath && fs.existsSync(resolvedPath)) {
         fileConfigPath = resolvedPath
+        break
       }
     }