Minimize cache size (closes #53)

Add --depth=1 to the git fetch command to minimize the git repositories size. A test showed that this reduced the cache size from 7507 MB to 962 MB.

.github/workflows/update.yml

@@ -24,7 +24,7 @@ jobs:
         path: ~/.cache
         # The cache is reset on the first build of every week this way.
         # Change the -1 part if you need to force reset the cache
-        key: "${{ steps.current-time.outputs.formattedTime }}-1"
+        key: "${{ steps.current-time.outputs.formattedTime }}-2"
     - uses: actions/checkout@v3
       name: Clone self repository
       with: