Fix macOS Big Sur builds on Cirrus

- Upgrade the Big Sur VM to use the Xcode 12.5 version. This has a newer
  version of brew installed on it that fixes an issue with an EOL package host
  that finally shut down for good recently.
- Use 'brew upgrade' for openssl and cmake, since those are both present on the
  base VM. This prevents 'brew install' from printing an error if the package
  exists but is out of date.

ci/macos/prepare.sh

@@ -5,4 +5,5 @@ sysctl hw.model hw.machine hw.ncpu hw.physicalcpu hw.logicalcpu
 set -e
 set -x
 
-brew install cmake swig openssl bison
+brew upgrade cmake openssl
+brew install swig bison