cli/command/plugin: remove uses of pkg/errors in tests

While there may be reasons to keep pkg/errors in production code,
we don't need them for these tests.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/plugin/upgrade_test.go

@@ -2,12 +2,12 @@ package plugin
 
 import (
 	"context"
+	"errors"
 	"io"
 	"testing"
 
 	"github.com/docker/cli/internal/test"
 	"github.com/docker/docker/api/types"
-	"github.com/pkg/errors"
 	"gotest.tools/v3/golden"
 )