[ci] Move CIRCT version to JSON file (#3472)

Change the way that the current CIRCT version is specified.  Previously,
this was code inside a GitHub Action.  Now this is data inside a JSON
file.  This is done to enable other GitHub actions to update the JSON
without having to have permissions to update workflows.

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