Bump cygwin/cygwin-install-action from 4 to 6

Bumps [cygwin/cygwin-install-action](https://github.com/cygwin/cygwin-install-action) from 4 to 6.
- [Release notes](https://github.com/cygwin/cygwin-install-action/releases)
- [Commits](cygwin/cygwin-install-action@v4...v6)

---
updated-dependencies:
- dependency-name: cygwin/cygwin-install-action
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/workflows/cygwin.yml

@@ -20,7 +20,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Set up Cygwin
-        uses: cygwin/cygwin-install-action@v4
+        uses: cygwin/cygwin-install-action@v6
         with:
           packages: cmake libboost-devel libtool gcc-g++ binutils make