Update the macOS image on Cirrus
Update the macOS image on Cirrus

We were being auto-upgraded. This commit makes the change known
and explicit.

cquinn/ponyc,sgebbie/ponyc,kulibali/ponyc,dipinhora/ponyc,sgebbie/ponyc,dipinhora/ponyc,jemc/ponyc,dipinhora/ponyc,kulibali/ponyc,ponylang/ponyc,cquinn/ponyc,cquinn/ponyc,ponylang/ponyc,sgebbie/ponyc,sgebbie/ponyc,kulibali/ponyc,kulibali/ponyc,jemc/ponyc,ponylang/ponyc,sgebbie/ponyc,jemc/ponyc,cquinn/ponyc