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/internal/hash/roller.go

@@ -0,0 +1,29 @@
+// 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 hash
+
+// Roller provides an interface for rolling hashes. The hash value will become
+// valid after hash has been called Len times.
+type Roller interface {
+	Len() int
+	RollByte(x byte) uint64
+}
+
+// Hashes computes all hash values for the array p. Note that the state of the
+// roller is changed.
+func Hashes(r Roller, p []byte) []uint64 {
+	n := r.Len()
+	if len(p) < n {
+		return nil
+	}
+	h := make([]uint64, len(p)-n+1)
+	for i := 0; i < n-1; i++ {
+		r.RollByte(p[i])
+	}
+	for i := range h {
+		h[i] = r.RollByte(p[i+n-1])
+	}
+	return h
+}