Improve nix devx + verify it in CI

[ghost]

The following changes were made:

• add 2 new CI jobs which run under a new Nix Buffers CI workflow (we also cache the nix store locally)
  -- nix flake check on ubuntu
  -- nix flake check on mac
• enable all CI workflows to be manually triggered from the actions panel
• clean up flake.nix
• speed up nix builds by providing libgit and openssl, instead of building from source

nix flake check now does the following:

• ensure all nix files are correctly formatted
• verify we can build buffrs under nix

[mara-schulke]

Except the comment RE gitignore, this lgtm!