Use 'buildkit' as default scope (#516)

This PR updates the schema to use the correct default value for `scope`
in the GHA caching options. This should be effectively a no-op.


From: https://docs.docker.com/build/cache/backends/gha/

<img width="892" alt="image"
src="https://github.com/user-attachments/assets/690cde79-e30e-4085-81fd-1c9dc1c6d7e9"
/>


Closes #496

provider/cmd/pulumi-resource-docker-build/schema.json

@@ -159,12 +159,7 @@
         "scope": {
           "type": "string",
           "description": "The scope to use for cache keys. Defaults to `buildkit`.\n\nThis should be set if building and caching multiple images in one\nworkflow, otherwise caches will overwrite each other.",
-          "default": "",
-          "defaultInfo": {
-            "environment": [
-              "buildkit"
-            ]
-          }
+          "default": "buildkit"
         },
         "token": {
           "type": "string",
@@ -389,12 +384,7 @@
         "scope": {
           "type": "string",
           "description": "The scope to use for cache keys. Defaults to `buildkit`.\n\nThis should be set if building and caching multiple images in one\nworkflow, otherwise caches will overwrite each other.",
-          "default": "",
-          "defaultInfo": {
-            "environment": [
-              "buildkit"
-            ]
-          }
+          "default": "buildkit"
         },
         "token": {
           "type": "string",