Do not copy .rmeta files into the sysroot of the build compiler during check

[Kobzol]

Before, when bootstrap did a check build of rustc stage N (with a build compiler that was stage N-1), it automatically copied the resulting .rmeta artifacts into the sysroot of the stage N-1 build compiler, so that stage N rustc_private tools such as miri could be compiled using the stage N-1 build compiler. This has a number of issues:

• It was done unconditionally, even if no rustc_private tools were actually built.
• If we did a check and a build of the same stage compiler in the same bootstrap invocation, the generated rmeta and rlib files could clash. This is also why you can see that check::Std actually doesn't copy the artifacts anymore (which forces us to build std instead of just checking it in a bunch of Check steps).
• It was polluting the sysroot of the build compiler. This is especially annoying for the stage 0 compiler, because we are forced to create an artificial sysroot for it, so that we can copy new stuff into it.
• It was very implicit in bootstrap.

Based on suggestions by @cuviper and @bjorn3, I tried to change how this behaves. Instead of copying the rmeta artifacts into the sysroot of the build compiler (from where they would be loaded implicitly), they are now stored in a separate transient bootstrap build directory, and they are then explicitly passed only when checking rustc_private tools using the -L flag. The flags are passed out-of-band through our rustc wrapper, to avoid invalidating the build cache.

Based on my local tests, this seemed to be working fine. If it works on CI, and we don't run into other issues after merging it, I'd like to do the same also for rlib artifacts generated during x build.

Based on #143816 (only the last commit is new).

r? @jieyouxu

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[bjorn3]

nit: s/hcekc/check in the PR title

[Kobzol]

I will also need to do this for the Clippy steps.

[Kobzol]

Uhh, x clippy ci ran clippy using in-tree clippy, not stage 0 clippy. I will have to go through the clippy steps first and make that more compatible with what x check does first.