Support for usage without `rustup`

[chrjabs]

Since pinning the toolchain version used for rustdoc in #267, cargo-rdme now depends on rustup being installed. I am aware that using Rust without having rustup installed is rather rare, and that the instability of the rustdoc JSON format requires some somewhat-quirky workarounds, but it would be great if it was possible to use cargo-rdme without having rustup installed.
I have two ideas for how this could be done:

1. (Probably the easiest fix for now.) Introduce a --use-default-toolchain-for-rustdoc (or something like that) flag. If this flag is set, simply always use the default active toolchain, it is then the user's responsibility that the rustdoc JSON version of that toolchain is compatible with cargo-rdme.
2. Allow for specifying a path to the cargo executable used for rustdoc invocations via a command line flag, or a environment variable. From skimming through the code a bit, I think this would require some changes to the rustdoc_json crate, since that crate, when no toolchain is explicitly specified, simply invokes cargo, and that behaviour currently can't be modified.

Let me know if any of these ideas sound interesting and if I could help with any of the implementation.

[orium]

Is installing rustup not an option? If not, can you explain why?

[chrjabs]

I'm on NixOS and I'm using project-specific pinned toolchains that are automatically loaded when I enter a project directory. While using rustup on NixOS is possible, it kind of goes against the philosophy of the distribution, which is why I'm trying to avoid it.
So far, all other tools that I'm using work with that setup, including cargo-semver-checks which also makes use of rustdoc JSON. But as I said, I'm aware that this is an unusual usecase and the fix shouldn't be a heavy maintenance burden for cargo-rdme.