Cypress tests: Create .git dir prior to deploy to Netlify

This is a workaround to netlify-cli's new way to resolve the project
directory. By creating an empty .git folder in the build directory,
netlify-cli deploys files relative to that directory (rather than
project root).

See: netlify/cli#953 (comment)

Author: FinnWoelm

cypress/fixtures/package.json

@@ -4,6 +4,7 @@
     "build": "../../../node_modules/.bin/next build",
     "postbuild": "node ../../../next-on-netlify",
     "preview": "../../../node_modules/.bin/netlify dev",
+    "predeploy": "mkdir -p .git",
     "deploy": "../../../node_modules/.bin/netlify deploy --json > deployment.json"
   }
 }