bug#61033: opam importer can't handle list field

[Csepp]

Truncated stack trace:

```
...
In guix/import/opam.scm:
    287:2  3 (opam->guix-package "mirage-crypto-pk" #:repo _ # _)
In unknown file:
           2 (filter #<procedure 7f43905afd00 at guix/import/opam.s…> …)
In guix/import/opam.scm:
   290:13  1 (_ ("mirage-no-solo5" "mirage-no-xen"))
In unknown file:
           0 (string-prefix? "conf-" ("mirage-no-solo5" "mirage-n…") …)

ERROR: In procedure string-prefix?:
In procedure string-prefix?: Wrong type argument in position 2 (expecting 
string): ("mirage-no-solo5" "mirage-no-xen")
```