[#607] *: Use keys.PublicKeys.Copy() where possible

Semantic patch:
```
@@
var dst identifier
var src identifier
var keys identifier
@@
 import keys "github.com/nspcc-dev/neo-go/pkg/crypto/keys"

-dst := make(keys.PublicKeys, len(src))
-copy(dst, src)
+dst := src.Copy()
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

cmd/frostfs-adm/internal/modules/morph/generate.go

@@ -104,8 +104,7 @@ func initializeWallets(v *viper.Viper, walletDir string, size int) ([]string, er
 	bftCount := smartcontract.GetDefaultHonestNodeCount(size)
 	for i := range wallets {
 		i := i
-		ps := make(keys.PublicKeys, len(pubs))
-		copy(ps, pubs)
+		ps := pubs.Copy()
 		errG.Go(func() error {
 			if err := addMultisigAccount(wallets[i], majCount, committeeAccountName, passwords[i], ps); err != nil {
 				return fmt.Errorf("can't create committee account: %w", err)

pkg/innerring/processors/governance/list_test.go

@@ -60,8 +60,7 @@ func TestNewAlphabetList(t *testing.T) {
 		orig := keys.PublicKeys{k[1], k[2], k[3], k[4]}
 		main := keys.PublicKeys{k[1], k[2], k[5], k[4]}
 
-		exp := make(keys.PublicKeys, len(main))
-		copy(exp, main)
+		exp := main.Copy()
 		sort.Sort(exp)
 
 		got, err := newAlphabetList(orig, main)