rustc-1.56.1-0: Specify Rust binaries to use for bootstrapping

From [Rust issue #90555][1]:

> Rustc is self-hosted, which means it needs a rust compiler to build
> itself so by default it downloads the previous version.
> If your distro already has a one-older rust compiler you can use that
> instead by adjusting your config.toml
>
> See the rustc-dev guide for an explanation of the build process.
>
> If you want to bootstrap from a C compiler you can start with mrustc,
> e.g. that's what guix does (they have updated their workflow since that
> blogpost).

[Config template][2]:

    # Instead of downloading the src/stage0.json version of Cargo specified, use
    # this Cargo binary instead to build all Rust code
    #cargo = "/path/to/cargo"

    # Instead of downloading the src/stage0.json version of the compiler
    # specified, use this rustc binary instead as the stage0 snapshot compiler.
    #rustc = "/path/to/rustc"

    # Instead of download the src/stage0.json version of rustfmt specified,
    # use this rustfmt binary instead as the stage0 snapshot rustfmt.
    #rustfmt = "/path/to/rustfmt"

[1]: https://github.com/rust-lang/rust/issues/90555
[2]: https://github.com/rust-lang/rust/blob/0b4ac62ddaf9ee01b8aaf9dd7097f1f541d64551/config.toml.example#L211-L221

rustc-1.56.1-0.build.sh

@@ -50,6 +50,10 @@ targets = "X86"
 experimental-targets = ""
 
 [build]
+cargo = "/usr/local/package/bin/cargo"
+rustc = "/usr/local/package/bin/rustc"
+rustfmt = "/usr/local/package/bin/rustfmt"
+
 # omit HTML docs to save time and space (comment this to build them)
 docs = false