ci/mac: pin homebrew for macOS 13 runner to last working version

since macOS 13 is now unsupported and homebrew also dropped support for
it too, the macOS 13 bottle downloads were removed. leading to either
a broken or very long dependency installation (build from source).

to fix, pin the last known working homebrew version for macOS 13 where
the bottle download links weren't removed yet. for this to work we need
to disable (auto) update of homebrew.

.github/workflows/build.yml

@@ -239,6 +239,7 @@ jobs:
           - os: "macos-13"
             arch: "intel"
             xcode: "Xcode_14.1"
+            homebrew: "2e821e2d8ae38540311cd6ec60f6e1b23d61d167"
           - os: "macos-14"
             arch: "arm"
             xcode: "Xcode_15.2"
@@ -263,7 +264,13 @@ jobs:
 
       - name: Install dependencies
         run: |
-          brew update
+          if [[ -n "${{ matrix.homebrew }}" ]]; then
+            export HOMEBREW_NO_AUTO_UPDATE=1
+            cd "$(brew --repo homebrew/core)"
+            git checkout --force ${{ matrix.homebrew }}
+          else
+            brew update
+          fi
           brew install -q autoconf automake pkgconf libtool python freetype fribidi little-cms2 \
             luajit libass ffmpeg meson rust uchardet mujs libplacebo molten-vk vulkan-loader vulkan-headers