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

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - Multiple exports are now allowed if the build daemon is detected to have
   version 0.13 of Buildkit or newer. (<https://github.com/pulumi/pulumi-docker-build/issues/21>)
+-  Use 'buildkit' as default scope (<https://github.com/pulumi/pulumi-docker-build/pull/516>)
 
 ### Changed