fixup! Improve CI caching

Author: ImSapphire

.github/workflows/c-cpp.yml

@@ -43,7 +43,9 @@ jobs:
       id: gen-cache-key
       shell: bash
       run: |
-        echo "cache-key=${{ matrix.os }}-$(git submodule foreach --recursive git rev-parse HEAD)-${{ hashFiles('CMakeLists.txt') }}" >> $GITHUB_OUTPUT
+        git submodule foreach --recursive git rev-parse HEAD > submodule_hashes.txt
+        echo "cache-key=${{ matrix.os }}-${{ hashFiles('submodule_hashes.txt') }}-${{ hashFiles('CMakeLists.txt') }}" >> $GITHUB_OUTPUT
+        rm submodule_hashes.txt
 
     - name: Set up CPM cache
       id: setup-cpm-cache