CI: fail cygwin build if we don't get the binaries

.github/workflows/cygwin_build.yml

@@ -27,6 +27,13 @@ jobs:
         run: |
           C:\Cygwin\bin\bash --login -c "Tools/scripts/cygwin_build.sh"
 
+      - name: Check build files
+        id: check_files
+        uses: andstor/file-existence-action@v1
+        with:
+          files: "artifacts/ArduPlane.elf.exe, artifacts/ArduCopter.elf.exe, artifacts/ArduHeli.elf.exe, artifacts/ArduRover.elf.exe, artifacts/ArduSub.elf.exe"
+          allow_failure: true
+
       - name: Archive build
         uses: actions/upload-artifact@v2
         with: