Toolchain.Target should not be serialized/cached by Hadrian

By caching the ./configure or ghc-toolchain generated Toolchain.Target in hadrian we run into awkard situations like #24199 (closed).

In !11621 (closed) we improve the error messages when the toolchain is stale, but, ultimately, the fix should be not to cache the Target, as noted in the MR by @mpickering.

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