Add missing property to schema

Author: neurolag

extensions-schema.json

@@ -12,6 +12,10 @@
         "type": "string",
         "description": "Repository URL to clone and publish from. If the extension publishes `.vsix` files as release artifacts, this will determine the repo to fetch the releases from."
       },
+      "checkout": {
+        "type": "string",
+        "description": "The commit to check out for building the extension"
+      },
       "location": {
         "type": "string",
         "description": "Location of the extension's package.json in the repository (defaults to the repository's root directory)"