[ci] Change CIRCT version to exist in one place

Change how the CIRCT version is specified for non-nightly flows to a
single environment variable in the ci.yml script.  This makes things
easier for either: (1) a human to update this or (2) a machine to update
this in a CD flow.

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