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/directcodec.go

@@ -0,0 +1,38 @@
+// 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
+
+// directCodec allows the encoding and decoding of values with a fixed number
+// of bits. The number of bits must be in the range [1,32].
+type directCodec byte
+
+// Bits returns the number of bits supported by this codec.
+func (dc directCodec) Bits() int {
+	return int(dc)
+}
+
+// Encode uses the range encoder to encode a value with the fixed number of
+// bits. The most-significant bit is encoded first.
+func (dc directCodec) Encode(e *rangeEncoder, v uint32) error {
+	for i := int(dc) - 1; i >= 0; i-- {
+		if err := e.DirectEncodeBit(v >> uint(i)); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Decode uses the range decoder to decode a value with the given number of
+// given bits. The most-significant bit is decoded first.
+func (dc directCodec) Decode(d *rangeDecoder) (v uint32, err error) {
+	for i := int(dc) - 1; i >= 0; i-- {
+		x, err := d.DirectDecodeBit()
+		if err != nil {
+			return 0, err
+		}
+		v = (v << 1) | x
+	}
+	return v, nil
+}