Change CI to use published CIRCT binary (#18)

Change GitHub CI to use a tagged, published Ubuntu binary of CIRCT
instead of relying on a nightly docker image.  This is both: (1) more
stable and (2) way faster.  This also removes a barrier to upstreaming
chisel-circt with chisel3.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>