vendor: update buildkit to v0.18.0-rc1

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/prometheus/common/model/labelset_string_go120.go

@@ -0,0 +1,39 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build !go1.21
+
+package model
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// String was optimized using functions not available for go 1.20
+// or lower. We keep the old implementation for compatibility with client_golang.
+// Once client golang drops support for go 1.20 (scheduled for August 2024), this
+// file can be removed.
+func (l LabelSet) String() string {
+	labelNames := make([]string, 0, len(l))
+	for name := range l {
+		labelNames = append(labelNames, string(name))
+	}
+	sort.Strings(labelNames)
+	lstrs := make([]string, 0, len(l))
+	for _, name := range labelNames {
+		lstrs = append(lstrs, fmt.Sprintf("%s=%q", name, l[LabelName(name)]))
+	}
+	return fmt.Sprintf("{%s}", strings.Join(lstrs, ", "))
+}