Checkout submodules and install lld linker (#17)

Summary:
There are the some missing steps to run buck2.  #18 needs this.

### Testing

Manually login to the runner, go to the docker container and run the following steps:

```
cd third-party/prelude
git checkout 971986683b987d399473969831bca6338833736d <-- the working prelude commit
buck2 build //test:size_test_all_ops
```

Pull Request resolved: #17

Test Plan: Testing on GitHub OSS CI https://github.com/pytorch/executorch/actions/runs/5651014889/job/15308367688?pr=17

Reviewed By: larryliu0820

Differential Revision: D47741878

Pulled By: huydhn

fbshipit-source-id: e8a698bd03cf2db5d9018448665571c42aa90400

Author: huydhn

.ci/docker/common/install_clang.sh

@@ -7,6 +7,8 @@ install_ubuntu() {
 
   apt-get install -y --no-install-recommends clang-"$CLANG_VERSION"
   apt-get install -y --no-install-recommends llvm-"$CLANG_VERSION"
+  # Also require LLD linker from llvm
+  apt-get install -y lld
 
   # Use update-alternatives to make this version the default
   update-alternatives --install /usr/bin/clang clang /usr/bin/clang-"$CLANG_VERSION" 50

.github/workflows/pull.yml

@@ -19,6 +19,7 @@ jobs:
       runner: linux.2xlarge
       docker-image: executorch-ubuntu-22.04-clang12
       fetch-depth: 0
+      submodules: 'true'
       ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
       script: |
         WORKSPACE=$(pwd)
@@ -28,6 +29,7 @@ jobs:
         ln -s "${WORKSPACE}" executorch
         # Install executorch
         source executorch/install.sh
+        popd
 
         # Just print out the list of packages for debugging
         pip list