flake.lock: Update

[github-actions]

Automated changes by the update-flake-lock GitHub Action.

Flake lock file updates:

• Updated input 'nixpkgs':
    'github:NixOS/nixpkgs/955079ad77917d544a88fcae9f6ca31738941254' (2022-02-06)
  → 'github:NixOS/nixpkgs/859c4a61a3d1cc8b32e535c0a8437532a340d1a4' (2022-03-20)

Running GitHub Actions on this PR

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To run GitHub Actions workflows on this PR, run:

git branch -D update_flake_lock_action
git fetch origin
git checkout update_flake_lock_action
git commit --amend --no-edit
git push origin update_flake_lock_action --force

[Mic92]

To quote @sterni regarding broken wasm target:

unfortunately the problem is nontrivial or rather requires some investigation
and the workaround is easy, but ugly
basically --sysroot changed somehow in cnojuction with our wrapper so it works with llvm 8 but not llvm 11
LLVM 8 -> 9 was the change it seems