Handle actions/checkout not supporting Cygwin

Per actions/checkout#767, the latest version of actions/checkout
overrides any existing global Git configuration, and instead uses its
own global configuration with safe.directory configured.  Unfortunately,
it configures a Windows path for safe.directory, which Cygwin Git
doesn't handle.  As a (hopefully) temporary workaround, pin that action
to the known-working version, and continue to set safe.directory
manually.

.github/workflows/ci.yml

@@ -19,12 +19,20 @@ jobs:
           platform: ${{ matrix.platform }}
           packages: git
         timeout-minutes: 10
-      - name: Configure safe.directory  # Workaround for actions/checkout#760
+
+      # Git v2.35.1 introduced safe.directory as a config option, which needs
+      # to be set on the GitHub runners.  The only git operation in this run is
+      # in actions/checkout, and as of v2.4.1, that action can handle the new
+      # Git config option.  Unfortunately, it's broken for Cygwin: see
+      # actions/checkout#767.  In the meantime, set the configuration manually,
+      # and use the old version of actions/checkout.
+      - name: Configure safe.directory
         run: git config --global --add safe.directory '*'
         timeout-minutes: 1
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v2.4.0
         timeout-minutes: 1
+
       - name: Load packages to install
         id: cyg-package-list
         run: |