bug#68895: Difference between trace behavior in guix repl and guile

[Adriel Dumas--Jondeau]

Here's what's happening in a guix repl:

```
(guix-user)> ,use (guix scripts build)
(guix-user)> ,trace (guix-build "hello") ; doesn't output any trace for some 
reason
```

And here'the same for a guile repl:

```
(guile-user)> ,use (guix scripts build)
(guile-user)> ,trace (guix-build "hello") ; however it does work
trace: |  (guix-build "hello")
trace: |  |  (_ (guix scripts) #:ensure #f)
trace: |  |  (_ #<procedure 7ff4802c5000 at ice-9/boot-9.scm:3241:7 ()>)
trace: |  |  |  (lock-mutex #<mutex 7ff487d01fc0>)
...
```

This was observed during a working group on profiling during the Guix Days.

-- 
Adriel

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