Fix `x test --stage 2 core` when download-rustc is enabled

This works by building std from source unconditionally instead of downloading it, for library tests only.

This was somewhat complicated because of the following requirements:
1. Unconditionally downloading libstd breaks `x test std`, because `coretests` requires the std loaded from the sysroot to match the std that's currently being tested.
2. Unconditionally rebuilding libstd breaks `x test ui-fulldeps librustdoc`, because anything loading `rustc_private` needs to use the same libstd that rustc was built with.

Break the knot by introducing a new `stage2-test-sysroot`, used only for testing `std` itself. This
holds a freshly compiled std, while `stage2` and `ci-rustc-sysroot` still hold the downloaded std.

This also extends the existing `cp_filtered` in Sysroot to apply to the `rust-std` component, not just the `rustc-dev` component.

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