DM-54126: Change to use shared buildx builder instead of recreating for each build

[stvoutsin]

Description

The tap-schema/build script creates and destroys a fresh buildx builder on every run.
build-all calls it 15 times and each time the new builder has to pull the mysql base image for 2 platforms from Docker.
That would lead to 30+ pulls per CI run including other pulls.

Fix

Create builder and delete it once in the build-all script.

Note: I've also changed the build script to also create it if it doesn't exist, to allow build to be run in isolation. If the intention is that this never should happen, we could remove that since the build-all script will always have created the builder.

Checklist

When making changes to YAML files in the schemas directory:

• If applicable, incremented the schema version number, following the guidelines in the contribution guide
• Referred to the documentation on specific schemas for additional versioning information, change constraints, or tasks that may need to be performed, based on which schema is being updated
• Ran Jenkins
• Added a news fragment describing the changes

[JeremyMcCormick]

Great, thanks for fixing!