ci: Bump macOS image to big-sur-xcode-12.5

This also removes the "brew update" added in commit
b738155.

.cirrus.yml

@@ -197,12 +197,11 @@ task:
 task:
   name: 'macOS 11 native [gui] [no depends]'
   brew_install_script:
-    - brew update
     - brew install boost libevent berkeley-db4 qt@5 miniupnpc libnatpmp ccache zeromq qrencode sqlite libtool automake pkg-config gnu-getopt
   << : *GLOBAL_TASK_TEMPLATE
   osx_instance:
     # Use latest image, but hardcode version to avoid silent upgrades (and breaks)
-    image: big-sur-xcode-12.4  # https://cirrus-ci.org/guide/macOS
+    image: big-sur-xcode-12.5  # https://cirrus-ci.org/guide/macOS
   env:
     << : *CIRRUS_EPHEMERAL_WORKER_TEMPLATE_ENV
     CI_USE_APT_INSTALL: "no"