Patch the thread_limit argument to Some(1) too

In the `text-ext4-casefold` jobs.

Author: EliahKagan

.github/workflows/ci.yml

@@ -242,11 +242,18 @@ jobs:
       - uses: taiki-e/install-action@v2
         with:
           tool: nextest
-      - name: More writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed reps
+      - name: Increase writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed reps
         run: |
           # Prepend leading digits "24" to the upper bound of the label range.
           sed -Ei 's/^(#\[test_matrix\(0\.\.=)([[:digit:]]+\)])$/\124\2/' \
             gix-worktree-state/tests/state/checkout.rs
+      - name: Modify the test code to use single-threaded checkout
+        run: |
+          # Replace the usual `thread_limit` argument with `Some(1)`.
+          sed -i 's/gix_features::parallel::num_threads(None).into()/Some(1)/' \
+            gix-worktree-state/tests/state/checkout.rs
+      - name: Display the changes made for this test
+        run: git diff
       - name: Test writes_through_symlinks_are_prevented_even_if_overwriting_is_allowed (nextest)
         run: |
           cargo nextest run -p gix-worktree-state-tests --no-default-features \