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/operation.go

@@ -0,0 +1,55 @@
+// 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 (
+	"fmt"
+	"unicode"
+)
+
+// operation represents an operation on the dictionary during encoding or
+// decoding.
+type operation interface {
+	Len() int
+}
+
+// rep represents a repetition at the given distance and the given length
+type match struct {
+	// supports all possible distance values, including the eos marker
+	distance int64
+	// length
+	n int
+}
+
+// Len returns the number of bytes matched.
+func (m match) Len() int {
+	return m.n
+}
+
+// String returns a string representation for the repetition.
+func (m match) String() string {
+	return fmt.Sprintf("M{%d,%d}", m.distance, m.n)
+}
+
+// lit represents a single byte literal.
+type lit struct {
+	b byte
+}
+
+// Len returns 1 for the single byte literal.
+func (l lit) Len() int {
+	return 1
+}
+
+// String returns a string representation for the literal.
+func (l lit) String() string {
+	var c byte
+	if unicode.IsPrint(rune(l.b)) {
+		c = l.b
+	} else {
+		c = '.'
+	}
+	return fmt.Sprintf("L{%c/%02x}", c, l.b)
+}