Add .cache dir to Turbo's cache (Shopify#7142)

An attempt at fixing the recent build errors surrounding:

> Cannot find module '../../.cache/site.json' or its corresponding type
declarations.

Not sure if this is the right strategy 🤷‍♀️ Tests for this solution have
passed 3 times, but it could be a fluke.

Author: martenbjork

turbo.json

@@ -3,7 +3,7 @@
   "baseBranch": "origin/main",
   "pipeline": {
     "build": {
-      "outputs": ["dist/**", ".next/**", "build/**"],
+      "outputs": ["dist/**", ".next/**", "build/**", ".cache/**"],
       "dependsOn": ["^build"]
     },
     "test": {