cue/load: do not format registry as a string
I saw an error message like this:
```
import failed: cannot get directory for external module "github.com/cue-labs-modules-testing" (registry &{%!q...): no dependency found for import path "github.com/cue-labs-modules-testing"
```
which is clearly wrong. Unfortunately `go vet` doesn't complain when `%q` is applied to interface arguments.
Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I2ae9fb9765da6274dd17ef643a9e0aca75a3a03d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1171754
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>