workflow: workaround homebrew issue

see Homebrew/homebrew-cask#150323

Author: drfiemost

.github/workflows/build.yml

@@ -34,6 +34,7 @@ jobs:
           HOMEBREW_NO_ANALYTICS: 1
           HOMEBREW_NO_INSTALL_CLEANUP: 1
         run: |
+          unset HOMEBREW_NO_INSTALL_FROM_API
           brew update
           brew upgrade || true
           brew install sdl2 sdl2_image sdl2_mixer libepoxy libopenmpt