Add dotenv-webpack to work around storybookjs/storybook#14403 (#821)

storybookjs/storybook#14497 and storybookjs/storybook#14403

tl;dr: Happo stopped working after #818 was merged, it turns out it is because
`npm run build-storybook` crashed due to the above issue(s).

Theoretically this is a workaround and can be removed after storybookjs/storybook#14403 is resolved, but there is no sign of when that'll happen

Deep technical reasons from the linked issue for why this fails:

> If npm hoists dotenv-webpack 6.0.4 to the node_modules root, everything works fine. However, if npm hoists dotenv-webpack 1.8, and the user has a .env file present, start-storybook fails to run.
>
> Unless the user is already pinning dotenv-webpack, the version that gets hoisted depends on how many dependencies are using 1.8 vs 6.0.4. (For example, in the repro steps below, if you remove @storybook/addon-essentials, npm hoists 6.0.4 and things work normally.) This makes for some extremely surprising and hard-to-pin-down behavior.

package.json

@@ -169,6 +169,7 @@
 		"concurrently": "^5.3.0",
 		"copy-webpack-plugin": "^6.4.1",
 		"css-loader": "^5.0.1",
+		"dotenv-webpack": "^7.0.3",
 		"error-overlay-webpack-plugin": "^0.4.2",
 		"eslint": "^7.16.0",
 		"eslint-config-airbnb": "^18.2.1",