GitHub-CI (cygwin): Bump cygwin/cygwin-install-action from 5 to 6.

* .github/workflows/make-cygwin.yaml: Bump cygwin/cygwin-install-action from 5
to 6.
See: #29

Author: dependabot[bot]

.github/workflows/make-cygwin.yaml

@@ -42,7 +42,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: install Cygwin build environment
-        uses: cygwin/cygwin-install-action@v5
+        uses: cygwin/cygwin-install-action@v6
         with:
           # The packages are listed in (alphabetically sorted) blocks:
           # The first block is for mandatory dependencies.