[clang-tools-extra] [clang-tidy] Enable plugin tests with LLVM_INSTALL_TOOLCHAIN_ONLY (PR #90370)

[Fangrui Song via cfe-commits]

MaskRay wrote:

This simplification seems desired. https://reviews.llvm.org/D111100#3302749 had a `LLVM_INSTALL_TOOLCHAIN_ONLY` condition, which seems unneeded.

https://github.com/llvm/llvm-project/pull/90370