v8.0.4-canary.3

Patches

• Drop images from experimental-amp and with-amp examples: #6446
• Fix for index.js pages inside a folder named 'index': #6423
• Update @zeit/next-preact example: #6390
• Remove cross-spawn: #6450
• Add error for exporting when target is not server: #6455

Credits

Huge thanks to @ijjk, @Janpot, and @toshi1127 for helping!