Read an xz release in Go, so the build seat needs no xz tool (hq issue 345)

Betterbird's pinned .tar.xz failed on the build seat with "this build seat has
no xz to read it": the seat's image is the catalogue's build-agent recipe,
which installs git and docker-cli alone, and xz had been added only to
cmd/mesh-builder/Dockerfile, which the seat does not build from. The stream
is now read with github.com/ulikunitz/xz (vendored), its dictionary capped
at xz's own largest preset, and what it unpacks to bounded as before; the
xz line in the unused Dockerfile is taken back out.

vendor/github.com/ulikunitz/xz/lzma/matchalgorithm.go

@@ -0,0 +1,52 @@
+// Copyright 2014-2022 Ulrich Kunitz. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package lzma
+
+import "errors"
+
+// MatchAlgorithm identifies an algorithm to find matches in the
+// dictionary.
+type MatchAlgorithm byte
+
+// Supported matcher algorithms.
+const (
+	HashTable4 MatchAlgorithm = iota
+	BinaryTree
+)
+
+// maStrings are used by the String method.
+var maStrings = map[MatchAlgorithm]string{
+	HashTable4: "HashTable4",
+	BinaryTree: "BinaryTree",
+}
+
+// String returns a string representation of the Matcher.
+func (a MatchAlgorithm) String() string {
+	if s, ok := maStrings[a]; ok {
+		return s
+	}
+	return "unknown"
+}
+
+var errUnsupportedMatchAlgorithm = errors.New(
+	"lzma: unsupported match algorithm value")
+
+// verify checks whether the matcher value is supported.
+func (a MatchAlgorithm) verify() error {
+	if _, ok := maStrings[a]; !ok {
+		return errUnsupportedMatchAlgorithm
+	}
+	return nil
+}
+
+func (a MatchAlgorithm) new(dictCap int) (m matcher, err error) {
+	switch a {
+	case HashTable4:
+		return newHashTable(dictCap, 4)
+	case BinaryTree:
+		return newBinTree(dictCap)
+	}
+	return nil, errUnsupportedMatchAlgorithm
+}