GitHub Actions Build directory

The point of this directory is to allow us to do a local installation of the rush tool, since

installing globally is not permitted on github actions
installing locally in the root directory of the repo creates node_modules there, and rush itself gives error messages since it thinks node_modules is not supposed to exist, since rush is supposed to be managing subproject dependencies.

Running rush from a subdirectory searches parent directories for rush.json and does the build starting from that file's location.