Rust plugin always installs the rustup snap, even when trying to use local cargo/rustc

[lengau]

Bug Description

Rustup always gets installed, even if we're trying to use local cargo and rustc installed from another part.

To Reproduce

Build with the following part

part yaml

parts:
  rust-deps:
    plugin: nil
    build-packages:
      - librust-indexmap-dev
      - librust-itoa-dev
      - librust-memchr-dev
      - librust-ryu-dev
      - librust-serde-dev
      - cargo
      - cargo-1.80
    override-pull: |
      ln -s /bin/cargo-1.80 /usr/local/bin/cargo
      ln -s /bin/rustc-1.80 /usr/local/bin/rustc
  librust-serde-json:
    plugin: rust
    after: [rust-deps]
    source: .

Relevant log output

2024-10-30 11:05:38.942 :: 2024-10-30 11:05:00.918 Initialising lifecycle
2024-10-30 11:05:38.942 :: 2024-10-30 11:05:00.918 part build packages: ['librust-indexmap-dev', 'librust-itoa-dev', 'librust-memchr-dev', 'librust-ryu-dev', 'librust-serde-dev', 'cargo', 'cargo-1.80']
2024-10-30 11:05:38.942 :: 2024-10-30 11:05:00.918 ignore patterns: []
2024-10-30 11:05:38.942 :: 2024-10-30 11:05:00.918 plugin build packages: {'gcc', 'pkg-config', 'curl', 'git', 'findutils'}
2024-10-30 11:05:38.942 :: 2024-10-30 11:05:00.919 plugin build snaps: {'rustup'}
2024-10-30 11:05:38.942 :: 2024-10-30 11:05:00.919 Installing build-packages
2024-10-30 11:05:38.942 :: 2024-10-30 11:05:00.919 Requested build-packages: ['cargo', 'cargo-1.80', 'curl', 'findutils', 'gcc', 'git', 'librust-indexmap-dev', 'librust-itoa-dev', 'librust-memchr-dev', 'librust-ryu-dev', 'librust-serde-dev', 'pkg-config']

<snip>

2024-10-30 11:05:38.945 :: 2024-10-30 11:05:29.870 Installing build-snaps
2024-10-30 11:05:38.945 :: 2024-10-30 11:05:30.141 Installing snap: rustup
2024-10-30 11:05:38.945 :: 2024-10-30 11:05:32.887 verify plugin environment for part 'rust-deps'
2024-10-30 11:05:38.945 :: 2024-10-30 11:05:32.890 verify plugin environment for part 'librust-serde-json'
2024-10-30 11:05:38.945 :: 2024-10-30 11:05:32.890 plugin validation environment: # Environment
2024-10-30 11:05:38.945 :: ## Application environment
2024-10-30 11:05:38.945 :: ## Part environment
2024-10-30 11:05:38.945 :: export CRAFT_ARCH_TRIPLET="x86_64-linux-gnu"
2024-10-30 11:05:38.945 :: export CRAFT_TARGET_ARCH="amd64"
2024-10-30 11:05:38.945 :: export CRAFT_ARCH_BUILD_ON="amd64"
2024-10-30 11:05:38.945 :: export CRAFT_ARCH_BUILD_FOR="amd64"
2024-10-30 11:05:38.945 :: export CRAFT_ARCH_TRIPLET_BUILD_ON="x86_64-linux-gnu"
2024-10-30 11:05:38.945 :: export CRAFT_ARCH_TRIPLET_BUILD_FOR="x86_64-linux-gnu"
2024-10-30 11:05:38.945 :: export CRAFT_PARALLEL_BUILD_COUNT="16"
2024-10-30 11:05:38.945 :: export CRAFT_PROJECT_DIR="/root/project"
2024-10-30 11:05:38.945 :: export CRAFT_STAGE="/root/stage"
2024-10-30 11:05:38.945 :: export CRAFT_PRIME="/root/prime"
2024-10-30 11:05:38.945 :: export CRAFT_PART_NAME="librust-serde-json"
2024-10-30 11:05:38.945 :: export CRAFT_STEP_NAME="BUILD"
2024-10-30 11:05:38.945 :: export CRAFT_PART_SRC="/root/parts/librust-serde-json/src"
2024-10-30 11:05:38.945 :: export CRAFT_PART_SRC_WORK="/root/parts/librust-serde-json/src"
2024-10-30 11:05:38.945 :: export CRAFT_PART_BUILD="/root/parts/librust-serde-json/build"
2024-10-30 11:05:38.945 :: export CRAFT_PART_BUILD_WORK="/root/parts/librust-serde-json/build"
2024-10-30 11:05:38.945 :: export CRAFT_PART_INSTALL="/root/parts/librust-serde-json/install"
2024-10-30 11:05:38.945 :: ## Plugin environment
2024-10-30 11:05:38.945 :: export PATH="${HOME}/.cargo/bin:${PATH}"
2024-10-30 11:05:38.945 :: ## User environment
2024-10-30 11:05:38.945 :: 2024-10-30 11:05:32.890 plugin validation command: 'cargo --version'
2024-10-30 11:05:38.945 :: 2024-10-30 11:05:33.068 Environment validation failed for part 'librust-serde-json': 'cargo' failed with error code 1.