Allow to specify cross version

[AnomalRoil]

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Motivation

Cross-compilation is super useful, but sadly cross is sometimes breaking things such as the compilation for powerpc64-unknown-linux-gnu and it doesn't really do semver when breaking stuff, but still has past versions we could use instead.

This project was already hit once: 4630dea

So I propose to be able to tag a cross version in the action.

Workflow example

  cross-test:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        rust_target:
          - powerpc-unknown-linux-gnu
          - powerpc64-unknown-linux-gnu
          - powerpc64le-unknown-linux-gnu
    timeout-minutes: 10
    steps:
      - uses: actions/checkout@v2
      - name: Install Rust nightly
        uses: actions-rs/toolchain@v1
        with:
            toolchain: nightly
            profile: minimal
            target: ${{ matrix.rust_target }}
            default: true
      - name: Build
        uses: actions-rs/cargo@v1
        with:
          use-cross: v0.1.16
          command: build
          args: --target ${{ matrix.rust_target }}
      - name: Test
        uses: actions-rs/cargo@v1
        with:
          use-cross: true
          command: test
          args: --target ${{ matrix.rust_target }}

When nothing is specified in the "use-cross" field, it would just use the existing cross version or download the latest, but if something is specified, it should fetch that version of cross instead.