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/crc.go

@@ -0,0 +1,54 @@
+// 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 xz
+
+import (
+	"hash"
+	"hash/crc32"
+	"hash/crc64"
+)
+
+// crc32Hash implements the hash.Hash32 interface with Sum returning the
+// crc32 value in little-endian encoding.
+type crc32Hash struct {
+	hash.Hash32
+}
+
+// Sum returns the crc32 value as little endian.
+func (h crc32Hash) Sum(b []byte) []byte {
+	p := make([]byte, 4)
+	putUint32LE(p, h.Hash32.Sum32())
+	b = append(b, p...)
+	return b
+}
+
+// newCRC32 returns a CRC-32 hash that returns the 64-bit value in
+// little-endian encoding using the IEEE polynomial.
+func newCRC32() hash.Hash {
+	return crc32Hash{Hash32: crc32.NewIEEE()}
+}
+
+// crc64Hash implements the Hash64 interface with Sum returning the
+// CRC-64 value in little-endian encoding.
+type crc64Hash struct {
+	hash.Hash64
+}
+
+// Sum returns the CRC-64 value in little-endian encoding.
+func (h crc64Hash) Sum(b []byte) []byte {
+	p := make([]byte, 8)
+	putUint64LE(p, h.Hash64.Sum64())
+	b = append(b, p...)
+	return b
+}
+
+// crc64Table is used to create a CRC-64 hash.
+var crc64Table = crc64.MakeTable(crc64.ECMA)
+
+// newCRC64 returns a CRC-64 hash that returns the 64-bit value in
+// little-endian encoding using the ECMA polynomial.
+func newCRC64() hash.Hash {
+	return crc64Hash{Hash64: crc64.New(crc64Table)}
+}