Unable to build unit tests without ROOT

I would have (maybe wrongly) expected that one should be able to build Core unit tests without ROOT.

But `cmake -B -S -DACTS_BUILD_UNITTESTS=ON` (suggested by the docs) fails without the additional
`-DACTS_BUILD_PLUGIN_ROOT=ON`.

Its not really a big deal (feel free to just close this issue). But I just wanted to bring this up because the docs don't mention it.

Also, being able to run Core unit tests without having to install extra plugins dependencies not needed to build Core itself makes it easier for users to add unit tests when contributing to Core.