ci: Update the macOS CI jobs to Big Sur

Homebrew stopped working for the Catalina-based images. After updating
to Big Sur and adding an explicit "brew update", the pipelines go green
again.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210716074616.1176282-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>