ci: macos: Choose python version

The CI base image used to have a python3 with headers etc installed in PATH,
but doesn't anymore. Instead of relying on a specific version in the base
image, explicitly install one ourselves.

On 16 and HEAD this lead to a build without python support, but on 15 CI
failed, due to explicitly enabled python3 support.

.cirrus.tasks.yml

@@ -471,8 +471,11 @@ task:
       openssl \
       p5.34-io-tty \
       p5.34-ipc-run \
+      python312 \
       tcl \
       zstd
+    # system python doesn't provide headers
+    sudo /opt/local/bin/port select python3 python312
     # Make macports install visible for subsequent steps
     echo PATH=/opt/local/sbin/:/opt/local/bin/:$PATH >> $CIRRUS_ENV
   upload_caches: macports