Fix `x.py clippy` to be less janky

[jyn514]

I hacked this together when I implemented it originally (#77351) and it breaks quite frequently because it uses the host toolchain instead of being managed by bootstrap.

I think ideally the fix would look something like:

• Download clippy for the bootstrap toolchain in bootstrap.py (look for add_component)
• Remove all this hacky code running rustc --version; treat it just the same as --cfg=bootstrap for build and check.
• Either put a todo!() for stages other than 0, or build tool::Clippy in-tree before running them, rather than the host clippy.

Originally posted by @jyn514 in #95766 (comment)

[jyn514]

@rustbot label +A-rustbuild +E-mentor +E-medium

[matthiaskrgr]

Note: right now an easy workaround that works 99% of the time for running x.py clippy is to just use clippy from the master toolchain (via rtim, switch master toolchain to the default one) and then run x.py clippy 😇

[jyn514]

Yes, that works. But it would be nice to not need the workaround.

(I think cargo +beta clippy should also work? but haven't tested in a while)