feat: Add BUILD_USER macro which returns the user who caused the build

[slide]

Adds a new macro that will return the username of the user who initiated the build. If a user did not initiate the build, then "" will be returned.

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[basil]

Duplicates functionality of https://plugins.jenkins.io/build-user-vars-plugin/ I suppose?

[slide]

Duplicates functionality of https://plugins.jenkins.io/build-user-vars-plugin/ I suppose?

That plugin does not provide tokens, so it may not be usable in all places that token-macro is used.