bug#61367: guix copy fails after copying with unauthorized public key

[Jonathan Brielmaier]

Hi,

I think it would be nice if `guix copy` would check for the public key
before actually copying data.

Currently it's like this:
$ guix copy --to=user@host /gnu/store/bla
guix copy: sending 439 store items (2.984 MiB) to 'host'...
[ takes some minutes ]
guix copy: error: unauthorized public key: (public-key
 (ecc
  (curve Ed25519)
  (q #KEYXYZ#)
  )
 )

It would be nice if it would fail upfront.

~Jonathan