: use git to get the prelude (#60)

Summary:


since where we are trying to put the prelude directory is already the root of a git repo, `buck2  init --git` is inappropriate. instead use git to get a copy.

Differential Revision:
D53099111

Privacy Context Container: L1124100

.github/workflows/buck-build-and-test.yml

@@ -25,5 +25,8 @@ jobs:
       - run: ./ocaml-setup.sh
       - run: cargo install --git https://github.com/facebookincubator/reindeer.git reindeer --force
       - run: reindeer --third-party-dir shim/third-party/rust buckify
+      - run: |-
+          git submodule add https://github.com/facebook/buck2-prelude.git prelude
+          git submodule update --init --depth 1
       - run: buck2 build shim//third-party/... -v 2
       - run: buck2 build root//... -v 2