wip: allow building of wasm targets

[snapbug]

WIP attempt(s) to address #494.

Create a new toolchain builder that uses the rustc-wasm32 package instead than the rustc package.

The rustc-wasm32 package doesn't provide rustdoc so I created a derivation to extract that from the regular rustc target. This shouldn't cause any issues.

Bazel selects the correct toolchain, but still errors out with the following:

❯ bazel build --config=nix //:wasm --verbose_failures
INFO: Analyzed target //:wasm (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
ERROR: /home/grawlix/repos/github.com/snapbug/rules_nixpkgs/examples/toolchains/rust/BUILD:11:12: Compiling Rust bin hello (1 files) failed: (Exit 1): process_wrapper failed: error executing command (from target //:hello)
  (cd /home/grawlix/.cache/bazel/_bazel_grawlix/e607e8e55d4b1c2cd07980276d90b4b7/sandbox/linux-sandbox/41/execroot/__main__ && \
  exec env - \
    CARGO_CFG_TARGET_ARCH=wasm32 \
    CARGO_CFG_TARGET_OS=unknown \
    CARGO_CRATE_NAME=hello \
    CARGO_MANIFEST_DIR='${pwd}' \
    CARGO_PKG_AUTHORS='' \
    CARGO_PKG_DESCRIPTION='' \
    CARGO_PKG_HOMEPAGE='' \
    CARGO_PKG_NAME=hello \
    CARGO_PKG_VERSION=0.0.0 \
    CARGO_PKG_VERSION_MAJOR=0 \
    CARGO_PKG_VERSION_MINOR=0 \
    CARGO_PKG_VERSION_PATCH=0 \
    CARGO_PKG_VERSION_PRE='' \
    SYSROOT=bazel-out/k8-fastbuild-ST-38b09129ed05/bin/external/nix_wasm_rust/rust_wasm_nix_impl \
  bazel-out/k8-opt-exec-C7777A24-ST-da94b34a892a/bin/external/rules_rust/util/process_wrapper/process_wrapper --subst 'pwd=${pwd}' -- bazel-out/k8-fastbuild-ST-38b09129ed05/bin/external/nix_wasm_rust/rust_wasm_nix_impl/bin/rustc hello.rs '--crate-name=hello' '--crate-type=bin' '--error-format=human' '--out-dir=bazel-out/k8-fastbuild-ST-38b09129ed05/bin' '--codegen=opt-level=0' '--codegen=debuginfo=0' '--remap-path-prefix=${pwd}=' '--emit=dep-info,link' '--color=always' '--target=wasm32-unknown-unknown' -L bazel-out/k8-fastbuild-ST-38b09129ed05/bin/external/nix_wasm_rust/rust_wasm_nix_impl/lib/rustlib/wasm32-unknown-unknown/lib -L bazel-out/k8-fastbuild-ST-38b09129ed05/bin/external/nix_wasm_rust/rust_wasm_nix_impl/lib/rustlib/x86_64-unknown-linux-gnu/lib '--edition=2021' '--sysroot=bazel-out/k8-fastbuild-ST-38b09129ed05/bin/external/nix_wasm_rust/rust_wasm_nix_impl')
# Configuration: 0590131d7658b76a08b19d13b2b4ddbfb70bcfab1a21c0745038366e0d666ffe
# Execution platform: @rules_nixpkgs_core//platforms:host

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
error: linker `rust-lld` not found
  |
  = note: No such file or directory (os error 2)

error: aborting due to previous error

Target //:wasm failed to build
INFO: Elapsed time: 0.168s, Critical Path: 0.08s
INFO: 5 processes: 5 internal.
FAILED: Build did NOT complete successfully

I think the way to make progress here is to do a similar thing as the rustdoc fix for rust-lld, although that might be a bit tougher as the rust-lld binary seems to be nested in the lib content. So have to investigate the full-path here, or alternatives.