release-macos: Avoid picking up libzstd from e.g. homebrew

As macOS itself doesn't ship with libzstd, avoid picking it up from
third party intallations.

CMake even looks for things in directories outside of $PATH;
for Darwin targets, CMake includes /opt/homebrew in
CMAKE_SYSTEM_PREFIX_PATH which gets searched automatically.

release-macos.sh

@@ -23,6 +23,10 @@ fi
 
 TAG=$1
 
+# macOS itself doesn't ship with libzstd; avoid picking up a zstd
+# dependency from libraries installed e.g. with homebrew.
+export LLVM_CMAKEFLAGS="-DLLVM_ENABLE_ZSTD=OFF"
+
 RELNAME=llvm-mingw-$TAG-ucrt-macos-universal
 DEST=$HOME/$RELNAME
 rm -rf $DEST