Enable wasm target in Rust toolchain in Nix Flake

[mpontus]

This is an attempt at enabling wasm build when building with nix build.

It enables wasm target in Rust toolchain using fenix.fromTemplateFile to create the toolchain from rust-toolchain.toml and add supported targets.

This fixes wasm32-unknown-unknown target error when building without SKIP_WASM_BUILD, but the build fails: with a different error: https://github.com/mpontus/centrifuge-chain/runs/8018805277?check_suite_focus=true

It fails because cargo can't vendor crates from a git source if they reference creates from a parent directory, such
cargo can't vendor crates from a git source if they reference crates from a parent directory, which appears to be the intended behavior: rust-lang/cargo#8885 (comment)

[mpontus]

cargo vendor is used by buildRustPackage to vendor crates for offline access during thebuild step: https://github.com/NixOS/nixpkgs/blob/1066f0ef92a2f869709eb0515694a1139f23065c/pkgs/build-support/rust/fetch-cargo-tarball/default.nix#L72

Alternative rust buildsers: crane and naersk also use it to vendor dependencies and consider it to be an upstream (cargo) issue:

• https://github.com/ipetkov/crane/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc
• Build fails for relative path crates from git dependency nix-community/naersk#260 (comment)

And the current consensus is calling it the indented behavior: With the current consus calling this the intended behavior: rust-lang/cargo#8885 (comment)

[mustermeiszer]

I close this for now. Am cleaning up a bit. Feel free to re-open if still needed.