Print key hash as hex in admin tool. (#8266)

The ProtoText printing of this structure prints the binary string as
escaped
utf8 text, which is essentially gibberish for my processes.

---------

Co-authored-by: Aaron Gable <aaron@letsencrypt.org>

cmd/admin/dryrun.go

@@ -32,10 +32,6 @@ type dryRunSAC struct {
 }
 
 func (d dryRunSAC) AddBlockedKey(_ context.Context, req *sapb.AddBlockedKeyRequest, _ ...grpc.CallOption) (*emptypb.Empty, error) {
-	b, err := prototext.Marshal(req)
-	if err != nil {
-		return nil, err
-	}
-	d.log.Infof("dry-run: %#v", string(b))
+	d.log.Infof("dry-run: Block SPKI hash %x by %s %s", req.KeyHash, req.Comment, req.Source)
 	return &emptypb.Empty{}, nil
 }

cmd/admin/key_test.go

@@ -178,6 +178,6 @@ func TestBlockSPKIHash(t *testing.T) {
 	err = a.blockSPKIHash(context.Background(), keyHash[:], u, "")
 	test.AssertNotError(t, err, "")
 	test.AssertEquals(t, len(log.GetAllMatching("Found 0 unexpired certificates")), 1)
-	test.AssertEquals(t, len(log.GetAllMatching("dry-run:")), 1)
+	test.AssertEquals(t, len(log.GetAllMatching("dry-run: Block SPKI hash "+hex.EncodeToString(keyHash[:]))), 1)
 	test.AssertEquals(t, len(msa.blockRequests), 0)
 }