Fix handling of rh-allow-sha1-signatures on Cygwin

The format of openssl.cnf has changed, so append to it instead of trying
to insert into it.  Test that openssl can sign RSA with SHA1 before
proceeding.

.github/workflows/c-cpp.yml

@@ -136,7 +136,8 @@ jobs:
       id: cygwin_install
       if: ${{ startsWith(matrix.target, 'windows') }}
       uses: cygwin/cygwin-install-action@a3d72946b163026bbd0fa9a88379ccbda4bd86bb # master
-    - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # main
+    - name: checkout openssh git repo
+      uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # main
     - name: setup CI system
       run: |
         sh -c "timeout 1200 .github/setup_ci.sh ${{ matrix.config }} ${{ matrix.target }} || .github/setup_ci.sh ${{ matrix.config }} ${{ matrix.target }}"