Bootstrap unit tests always require cmake

[Noratrieb]

---- core::builder::tests::dist::llvm_out_behaviour stdout ----

Couldn't find required command: cmake

You should install cmake, or set `download-ci-llvm = true` in the
`[llvm]` section of `config.toml` to download LLVM rather
than building it.

thread 'core::builder::tests::dist::llvm_out_behaviour' panicked at /home/nora/projects/rust2/src/tools/build_helper/src/util.rs:18:9:
status code: 1

It mentions using download-ci-llvm, but that doesn't help, it requires cmake. The message should probably mention that if possible.