🩹 Fix git warnings in Github workflows

The "Checkout" action is resulting in warnings that Git is returning
nonzero exit-codes, and this is due to the lack of the `.git/module`
registration for cached 3rd-party submodules. The easy fix here is for
the cache to also include the `.git/modules` path in addition to the
existing `3rd-party` directory -- since this will enable the remote
path to be discovered and hopefully not fail.

.github/actions/checkout/action.yaml

@@ -8,8 +8,10 @@ runs:
       id: cache-submodule
       uses: actions/cache@v3
       with:
-        path: 3rd-party
-        key: submodules-${{ hashFiles('.gitmodules')}}
+        path: |
+          3rd-party
+          .git/modules
+        key: submodules-${{ hashFiles('.gitmodules')}}-v2
         restore-keys: submodules-
 
     - name: Update git submodules