The problem was a locked method calling backend(), thus calling
ensure_uniqueness(), which in turn waited for a lock. The solution is to
first ensure uniqueness and obtain, then lock, then use a reference on the
unique backend. This is in general acchieved by
```
auto& backend_ref = backend(); // calles ensure_uniqueness()
std::lock...
// work with backend_ref
```

To make it a bit more explicit what is going on we could also let
ensure_uniqueness() return a `BackendType&` and use that instead of
backend().

Since those could not be guaranteed to be thread safe.

And require it in ContainerInterface. This will allow interface default
implementations to call ensure_uniqueness() only one in a for loop.

That commit broke test discovery for me.

This reverts commit ac665edb.

The FindEigen script's variable name had changed so these tests haven't
beeen built in a while.
Also disables trying to compile eingemmappeddense for complex fields.

- macro is not a function

* do not hide get_entry_ref in derived classes
* use CRTP for operator[] (this will facilitate athread safe implementation
  at some point).
* implement operator[] by get_entry_ref only in derived classes

Add missing traits + fix Stuff usage

- run it only once across builders
- set binary name via git-config.env and install it too

we need this for our clang format status report script