Use load instead of push to support remote builders

[bluesliverx]

Description

Adds support for building on remote builders that cannot access the local registry.

Related Issue

Motivation and Context

When using remote builders, they cannot access the temporary local registry since it runs on localhost. Attempting to do with fully qualified names is also problematic unless we set up full certs, auth, etc. To prevent this, we can simply --load instead of --push'ing in the buildx build, then push the resulting image for the final manifest creation.

How Has This Been Tested?

I tested this in a development environment to ensure it does as expected.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.