[build:web] jimp cannot generate images with a default resize mode

[EvanBacon]

evanbacon@Evans-MBP b10 % expod build:web -c

✖ Expo Webpack
  Compiled with some errors in 3.01s

› Favicon: No template image found, skipping auto generation...
› Safari Icons: No template image found, skipping auto generation...

Using node to generate images. This is much slower than using native packages.
› Optionally you can stop the process and try again after successfully running `npm install -g sharp-cli`.
› If you are using `expo-cli` to build your project then you could use the `--no-pwa` flag to skip the PWA asset generation step entirely.
› Chrome Icons: No template image found, skipping auto generation...
 web  Failed to compile.

Unsupported fit: undefined. Please choose either 'cover', or 'contain' when using Jimp
Error: Unsupported fit: undefined. Please choose either 'cover', or 'contain' when using Jimp
    at /@expo/xdl@57.9.14/src/Webpack.ts:281:23
    at finalCallback (/Users/evanbacon/Documents/GitHub/cli/node_modules/webpack/lib/Compiler.js:257:39)
    at /Users/evanbacon/Documents/GitHub/cli/node_modules/webpack/lib/Compiler.js:306:14
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/evanbacon/Documents/GitHub/cli/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:31:1)
    at AsyncSeriesHook.lazyCompileHook (/Users/evanbacon/Documents/GitHub/cli/node_modules/tapable/lib/Hook.js:154:20)
    at /Users/evanbacon/Documents/GitHub/cli/node_modules/webpack/lib/Compiler.js:304:22
    at Compiler.emitRecords (/Users/evanbacon/Documents/GitHub/cli/node_modules/webpack/lib/Compiler.js:499:39)
    at /Users/evanbacon/Documents/GitHub/cli/node_modules/webpack/lib/Compiler.js:298:10
    at /Users/evanbacon/Documents/GitHub/cli/node_modules/webpack/lib/Compiler.js:485:14
    at _next1 (eval at create (/Users/evanbacon/Documents/GitHub/cli/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:19:1)
    at eval (eval at create (/Users/evanbacon/Documents/GitHub/cli/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:30:1)
    at /Users/evanbacon/Documents/GitHub/cli/node_modules/copy-webpack-plugin/dist/index.js:116:7
    at _next0 (eval at create (/Users/evanbacon/Documents/GitHub/cli/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:26:1)
    at eval (eval at create (/Users/evanbacon/Documents/GitHub/cli/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:41:1)
    at /Users/evanbacon/Documents/GitHub/cli/node_modules/copy-webpack-plugin/dist/index.js:116:7
    at AsyncSeriesHook.eval [as callAsync] (eval at create (/Users/evanbacon/Documents/GitHub/cli/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:37:1)

We should ensure a default value is defined. Maybe we should also have an environment variable for skipping sharp CLI.