Draft: Development shell with Nix

Problem: Redox can't be built in pure way

Solution: Development shell that uses dependencies from distro-agnostic repository, rather than using distro-specific package manager.

Changes introduced by this pull request:

• flake.nix has been added.
• depends.mk now ignores non-existence of rustup inside nix shell
  • Rust toolchain is installed with Nix

Drawbacks: [if any, describe the drawbacks of this pull request.]

TODOs:

• make all doesn't work (yet), looks like Redox build systems wants something from rustup.

Fixes: [what issues this fixes.]

State: WIP/Draft

Blocking/related: [issues or PRs blocking or being related to this issue.]

Other: [optional: for other relevant information that should be known or cannot be described in the other fields.]

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