Check out the entire repository

One of the build steps, at least for releases, is to push the tag to the
Cygwin Git repositories.  That's not going to be possible from a shallow
clone, so use a full clone instead.  It's not as if this is ever going
to be a large repository!

.github/workflows/ci.yml

@@ -16,6 +16,8 @@ jobs:
         timeout-minutes: 10
       - name: Checkout
         uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
         timeout-minutes: 1
       - name: Load packages to install
         id: cyg-package-list