ci: checkout mulle-core[-all-load] into stash/ explicitly

The recursive git-submodule setup that populated stash/mulle-core and
stash/mulle-core-all-load was removed, leaving the two dependency build
steps in cmake-errors.yml pointing at working-directories that no longer
existed (CI failed on all platforms: 'working directory .../stash/mulle-core:
No such file or directory'). Replace 'submodules: recursive' with explicit
actions/checkout steps that clone both deps into stash/ on the same ref
(github.ref_name) as this build.

Author: mulle-nat

.github/workflows/cmake-errors.yml

@@ -61,8 +61,25 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
+
+      # mulle-core and mulle-core-all-load used to be recursive git submodules
+      # under stash/. That recursive-submodule setup was removed, which left the
+      # two "build ... from stash/" steps below dangling (their working-directory
+      # no longer existed). Check them out explicitly instead, from the same ref
+      # as this build (github.ref_name, e.g. prerelease / develop / master).
+      - name: Checkout mulle-core into stash
+        uses: actions/checkout@v4
+        with:
+          repository: mulle-core/mulle-core
+          ref: ${{ github.ref_name }}
+          path: stash/mulle-core
+
+      - name: Checkout mulle-core-all-load into stash
+        uses: actions/checkout@v4
         with:
-          submodules: recursive
+          repository: mulle-core/mulle-core-all-load
+          ref: ${{ github.ref_name }}
+          path: stash/mulle-core-all-load
 
       # Set installation directory based on OS
       - name: Set install directory