test(@angular-devkit/build-angular): disable html karma coverage reporter

Under bazel this causes the below error.

```
Unexpected error while generating coverage report. [Error: EEXIST: file already exists, mkdir
```

Author: alan-agius4

packages/angular_devkit/build_angular/src/builders/karma/tests/behavior/code-coverage_spec.ts

@@ -88,6 +88,8 @@ describeBuilder(execute, KARMA_BUILDER_INFO, (harness) => {
     });
 
     it('should remapped instrumented code back to the original source', async () => {
+      await harness.modifyFile('karma.conf.js', (content) => content.replace('lcov', 'html'));
+
       await harness.modifyFile('src/app/app.component.ts', (content) => {
         return content.replace(
           `title = 'app'`,

packages/angular_devkit/build_angular/test/hello-world-app/karma.conf.js

@@ -33,7 +33,6 @@ module.exports = function(config) {
       dir: path.join(__dirname, './coverage'),
       subdir: '.',
       reporters: [
-        {type: 'html'},
         {type: 'lcov'},
       ],
     },