CI: Try to build and install stage2, to test mathlib without a stage0 update on the branch

Author: nomeata

.github/workflows/build-template.yml

@@ -152,10 +152,10 @@ jobs:
           fi
           # contortion to support empty OPTIONS with old macOS bash
           cmake .. --preset ${{ matrix.CMAKE_PRESET || 'release' }} -B . ${{ matrix.CMAKE_OPTIONS }} ${OPTIONS[@]+"${OPTIONS[@]}"} -DLEAN_INSTALL_PREFIX=$PWD/..
-          time make -j$NPROC
+          time make stage2 -j$NPROC
       - name: Install
         run: |
-          make -C build install
+          make -C build/stage2 install
       - name: Check Binaries
         run: ${{ matrix.binary-check }} lean-*/bin/* || true
       - name: Count binary symbols