Fix fuchsia target triple to unbreak docs.rs build

[dtolnay]

What does this PR do

Fuchsia timeline:

• nix <0.20.0: no support for fuchsia
• nix 0.20.0 – 0.26.1: successful docs for x86_64-fuchsia
  • https://docs.rs/nix/0.20.0/x86_64-fuchsia/nix/index.html
  • https://docs.rs/nix/0.26.1/x86_64-fuchsia/nix/index.html
• Rust 1.68.0 renames x86_64-fuchsia to x86_64-unknown-fuchsia
  • Add vendor to Fuchsia's target triple rust-lang/rust#106429
  • Accept old spelling of Fuchsia target triples rust-lang/rust#106636
• nix 0.26.2 – 0.29.0: no build for x86_64-fuchsia, but successful for everything else
  • failure: https://docs.rs/crate/nix/0.29.0/builds/1230030/x86_64-fuchsia.txt

    adding target x86_64-fuchsia for toolchain nightly
    running `Command { std: CARGO_HOME="/home/cratesfyi/workspace-builder/cargo-home" RUSTUP_HOME="/home/cratesfyi/workspace-builder/rustup-home" "/home/cratesfyi/workspace-builder/cargo-home/bin/rustup" "target" "add" "--toolchain" "nightly" "x86_64-fuchsia", kill_on_drop: false }`
    [stderr] error: toolchain 'nightly-x86_64-unknown-linux-gnu' does not support target 'x86_64-fuchsia'
    [stderr] note: you can see a list of supported targets with `rustc --print=target-list`
    [stderr] note: if you are adding support for a new target to rustc itself, see https://rustc-dev-guide.rust-lang.org/building/new-target.html

• Rust 1.83.0 deletes support for x86_64-fuchsia
  • Remove x86_64-fuchsia and aarch64-fuchsia target aliases rust-lang/rust#130657
• nix 0.30.0: no docs for anything
  • https://docs.rs/crate/nix/0.30.0/builds/2054018

    # pre-build errors
    command failed: exit status: 101

    error: failed to run `rustc` to learn about target-specific information

    Caused by:
    process didn't exit successfully: `/home/cratesfyi/workspace/rustup-home/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc - --crate-name ___ --print=file-names --target x86_64-fuchsia --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit status: 1)
    --- stderr
    error: error loading target specification: could not find specification for target "x86_64-fuchsia"
    |
    = help: run `rustc --print target-list` for a list of built-in targets

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[asomers]

Thanks @dtolnay ! We'll hurry out a patch release to fix docs.rs.