chore(build): Update build schema

[rhysparry]

Recent Nuke.Build update impacts the build schema.

This pull request includes changes to the .nuke/build.schema.json file to restructure the JSON schema and improve its organization. The most important changes include moving properties into an allOf array and referencing the NukeBuild definition.

Schema restructuring:

• Moved the Configuration, GithubRef, NuGetApiKey, Solution, and Version properties into an allOf array. [1] [2]
• Added a reference to the NukeBuild definition within the allOf array.

[rhysparry]

This change is part of the following stack:

• chore(build): Update build schema #42 ◀

_{Change managed by git-spice.}