Tools: Unify github build workflow and sync scripts for new block theme

[ryelle]

Currently there are two build workflows — build.yml to build the plugins and classic theme, and build-blocks.yml which only builds the new block theme. These also push to separate git branches (build and build-blocks).

Likewise, there are two sync scripts — the original for the classic theme and plugins, and a new one for the block theme.

These should be combined into one workflow, one branch, and one sync script.