: 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 as a submodule~~ delegate to `dtolnay/install-buck2@latest` to pull in  the prelude submodule (at a coherent version with the buck2 installed)

Differential Revision:
D53099111

Privacy Context Container: L1124100

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

@@ -15,6 +15,8 @@ jobs:
       - uses: actions/checkout@v4
       - uses: Swatinem/rust-cache@v2
       - uses: dtolnay/install-buck2@latest
+        with:
+          prelude-submodule: prelude
       - run: sudo apt-get update && sudo apt-get install opam
         if: matrix.os == 'ubuntu'
       - run: brew install opam