buck2: unstable-2024-01-15 -> unstable-2024-03-15
https://github.com/facebook/buck2/releases/tag/2024-03-15
This is needed to use Buck2 from nixpkgs with the latest
prelude, due to needing the `remote_execution_dependencies` parameter
for https://github.com/facebook/buck2-prelude/commit/99c1bcf441c1edc6aa568a6334fa61c182890f68.
Among other things, updating Buck2 adds support for the
`remote_execution`
authored by Ben Navetta and committed by Austin Seipp f8399b22 55a2b963