Switch github runner to macos-13.

These runners are based on Homebrew, and Homebrew appears to have
dropped support for macos versions less than 12. That means the
macos-11 runner now fails every time trying to build qt5 from source.
Switch away from the macos-11 runner to a newer runner that is still
supported by Homebrew.

.github/workflows/buildmaster.yml

@@ -11,7 +11,7 @@ jobs:
     name: build
     strategy:
       matrix:
-        os: ['ubuntu-20.04', 'ubuntu-22.04', 'macos-11', 'macos-12']
+        os: ['ubuntu-20.04', 'ubuntu-22.04', 'macos-12', 'macos-13']
         cc: ['gcc', 'clang']
         include:
           - cc: 'gcc'