fix(rollup): allow config files to override default onwarn method

Fixes #2084

packages/rollup/index.js

@@ -103,12 +103,16 @@ function extractEnvironmentVariables(vars) {
 //  input:  https://rollupjs.org/guide/en/#inputoptions-object
 //  output: https://rollupjs.org/guide/en/#outputoptions-object
 async function parseCLIArgs(args) {
-  let inputOptions = {
+  // Options which the CLI args or config file can override
+  const defaultInputOptions = {
     onwarn(...warnArgs) {
       worker.log(...warnArgs);
     },
   };
 
+  // Options which can override the config file
+  let inputOptions = {};
+
   let outputOptions = {};
 
   let configFile = null;
@@ -194,6 +198,9 @@ async function parseCLIArgs(args) {
     delete inputOptions.output;
   }
 
+  // Provide default inputOptions which can be overwritten
+  inputOptions = {...defaultInputOptions, ...inputOptions};
+
   // The inputs are the rule entry_point[s]
   inputOptions.input = inputs;