WIP: github: Use a recent release of the toolchain

Author: mstorsjo

.github/workflows/test-libcxx.yml

@@ -55,17 +55,9 @@ jobs:
       - name: Install dependencies
         run: |
           choco install ninja
-      - name: Download toolchain
-        uses: dawidd6/action-download-artifact@v6
-        with:
-          workflow: build.yml
-          workflow_conclusion: success
-          commit:
-          branch: master
-          event: schedule
-          name: windows-ucrt-${{matrix.arch}}-toolchain
       - name: Unpack toolchain
         run: |
+          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20250514/llvm-mingw-20250514-ucrt-${{matrix.arch}}.zip
           Expand-Archive llvm-mingw-*.zip -DestinationPath .
           del llvm-mingw-*.zip
           mv llvm-mingw-* c:\llvm-mingw