Switch homebrew build env to new xcode 11.4

default 11.3.1 in the github image is causing brew to fail with "outdated xcode" message

.github/workflows/nightly-Homebrew-build.yml

@@ -21,6 +21,7 @@ jobs:
   build-OpenBLAS-with-Homebrew:
     runs-on: macos-latest
     env:
+      DEVELOPER_DIR: /Applications/Xcode_11.4.app/Contents/Developer
       HOMEBREW_DEVELOPER: "ON"
       HOMEBREW_DISPLAY_INSTALL_TIMES: "ON"
       HOMEBREW_NO_ANALYTICS: "ON"