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/bits.go

@@ -0,0 +1,79 @@
+// 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 (
+	"errors"
+	"io"
+)
+
+// putUint32LE puts the little-endian representation of x into the first
+// four bytes of p.
+func putUint32LE(p []byte, x uint32) {
+	p[0] = byte(x)
+	p[1] = byte(x >> 8)
+	p[2] = byte(x >> 16)
+	p[3] = byte(x >> 24)
+}
+
+// putUint64LE puts the little-endian representation of x into the first
+// eight bytes of p.
+func putUint64LE(p []byte, x uint64) {
+	p[0] = byte(x)
+	p[1] = byte(x >> 8)
+	p[2] = byte(x >> 16)
+	p[3] = byte(x >> 24)
+	p[4] = byte(x >> 32)
+	p[5] = byte(x >> 40)
+	p[6] = byte(x >> 48)
+	p[7] = byte(x >> 56)
+}
+
+// uint32LE converts a little endian representation to an uint32 value.
+func uint32LE(p []byte) uint32 {
+	return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 |
+		uint32(p[3])<<24
+}
+
+// putUvarint puts a uvarint representation of x into the byte slice.
+func putUvarint(p []byte, x uint64) int {
+	i := 0
+	for x >= 0x80 {
+		p[i] = byte(x) | 0x80
+		x >>= 7
+		i++
+	}
+	p[i] = byte(x)
+	return i + 1
+}
+
+// errOverflow indicates an overflow of the 64-bit unsigned integer.
+var errOverflowU64 = errors.New("xz: uvarint overflows 64-bit unsigned integer")
+
+// readUvarint reads a uvarint from the given byte reader.
+func readUvarint(r io.ByteReader) (x uint64, n int, err error) {
+	const maxUvarintLen = 10
+
+	var s uint
+	i := 0
+	for {
+		b, err := r.ReadByte()
+		if err != nil {
+			return x, i, err
+		}
+		i++
+		if i > maxUvarintLen {
+			return x, i, errOverflowU64
+		}
+		if b < 0x80 {
+			if i == maxUvarintLen && b > 1 {
+				return x, i, errOverflowU64
+			}
+			return x | uint64(b)<<s, i, nil
+		}
+		x |= uint64(b&0x7f) << s
+		s += 7
+	}
+}