corpus: remove submodule

A Git submodule was introduced in commit 341ac1e
("corpus: initial integration"). The main idea was to keep in sync
source code of tests and their seed corpus.
However, this approach is not convenient:

- seed corpus updates often and someone need to regularly bump
  submodule version, otherwise outdated seed corpus is used
- seed corpus takes about 5Gb and git clone of repository with
  tests takes significant time

Author: ligurio

.clusterfuzzlite/Dockerfile

@@ -12,6 +12,5 @@ RUN apt-get update && apt-get install -y \
 
 COPY . $SRC/lua-c-api-tests
 WORKDIR $SRC/lua-c-api-tests
-RUN rm -rf corpus
 RUN git clone --depth 1 --branch cfl https://github.com/ligurio/lua-c-api-corpus corpus
 COPY .clusterfuzzlite/build.sh $SRC/

.github/workflows/test.yaml

@@ -37,9 +37,15 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
+
+      - name: Download seed corpus
+        uses: actions/checkout@v4
         with:
+          repository: ligurio/lua-c-api-corpus
+          # The tests expect a corpus directory in a repository
+          # root directory. Otherwise corpus will not be used.
+          path: corpus
           fetch-depth: 0
-          submodules: recursive
 
       - name: Setup common packages
         run: |