ci: don't link with libzstd in release builds

libzstd was added in LLVM 15, but we don't currently use it. So let's
disable it in LLVM just like libzlib.

See: https://reviews.llvm.org/D128465

.github/workflows/build-macos.yml

@@ -47,7 +47,7 @@ jobs:
         uses: actions/cache@v3
         id: cache-llvm-build
         with:
-          key: llvm-build-15-macos-v2
+          key: llvm-build-15-macos-v3
           path: llvm-build
       - name: Build LLVM
         if: steps.cache-llvm-build.outputs.cache-hit != 'true'