CI: bump "actions/checkout" to v3 for cross zoo matrix

actions/checkout@v2 is deprecated, accidently it was not updated in our
build definition
diff --git a/.github/workflows/cross-zoo.yml b/.github/workflows/cross-zoo.yml
index e2a5816..f2c8d7a 100644
--- a/.github/workflows/cross-zoo.yml
+++ b/.github/workflows/cross-zoo.yml
@@ -97,7 +97,7 @@
         sudo apt-get -yq --force-yes install \
             gcc-${{ matrix.platform.arch }} \
             ${{ matrix.platform.libs }}
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
 
 
     - name: install quictls