bug#58495: opam import generates wrong check phase

[Csepp]

So I'm working on building MirageOS unikernels with Guix, which means
using the OPAM importer *a lot*, which unearths some very Fun TM bugs.
Here is the first one:

guix import opam mirage-clock

The correct check phase is this:
```
(replace 'check
           (lambda* (#:key tests? #:allow-other-keys)
             (invoke "dune" "runtest" "--release")))
```