chore: fix site deploys (#8878)

force Vercel to rebuild sharp everytime - it's prone to breaking at some point and we'd have to clear the cache

sites/svelte.dev/vercel.json

@@ -3,5 +3,5 @@
 	"github": {
 		"silent": true
 	},
-	"buildCommand": "cd ../../packages/svelte && pnpm prepublishOnly && cd ../../sites/svelte.dev && pnpm build"
-}
+	"buildCommand": "cd ../../packages/svelte && pnpm prepublishOnly && cd ../../sites/svelte.dev && pnpm rebuild sharp && pnpm build"
+}