Pin cygwin-install-action to v6.1 by commit SHA

Replace @master reference with the pinned commit SHA for v6.1,
and remove cygwin.yml from the zizmor unpinned-uses ignore list
since it is now properly pinned.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/workflows/cygwin.yml

@@ -45,7 +45,7 @@ jobs:
           persist-credentials: false
 
       - name: Setup Cygwin
-        uses: cygwin/cygwin-install-action@master
+        uses: cygwin/cygwin-install-action@711d29f3da23c9f4a1798e369a6f01198c13b11a # v6.1
         with:
           packages: ruby gcc-core make autoconf libtool libssl-devel libyaml-devel libffi-devel zlib-devel rubygems
           site: |

.github/zizmor.yml

@@ -28,5 +28,4 @@ rules:
     disable: true
   unpinned-uses:
     ignore:
-      - cygwin.yml
       - wsl.yml