Set CYGWIN at top-level.

The ensures the symlinks created in any step (eg, logs) are native
symlinks and can be understood by the log upload step.

.github/workflows/c-cpp.yml

@@ -124,6 +124,7 @@ jobs:
     runs-on: ${{ matrix.target }}
     env:
       EPHEMERAL_VM: yes
+      CYGWIN: "winsymlinks:native"
     steps:
     - name: check RUN_ONLY_TARGET_CONFIG
       if: vars.RUN_ONLY_TARGET_CONFIG != ''
@@ -135,8 +136,6 @@ jobs:
       id: cygwin_install
       if: ${{ startsWith(matrix.target, 'windows') }}
       uses: cygwin/cygwin-install-action@a3d72946b163026bbd0fa9a88379ccbda4bd86bb # master
-      env:
-        CYGWIN: "winsymlinks:native"
     - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # main
     - name: setup CI system
       run: |