[ci] Add "nightly" option to install-circt action

Add support for grabbing the latest nightly version of `firtool` to the
install-circt action.  This requires passing the "github.token" so that
the GitHub API can be used.  Note that the token is marked as "required",
however, it is only actually needed when using a nightly build.

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