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/.gitignore

@@ -0,0 +1,28 @@
+# .gitignore
+
+TODO.html
+README.html
+
+lzma/writer.txt
+lzma/reader.txt
+
+cmd/gxz/gxz
+cmd/xb/xb
+
+# test executables
+*.test
+
+# profile files
+*.out
+
+# vim swap file
+.*.swp
+
+# executables on windows
+*.exe
+
+# default compression test file
+enwik8*
+
+# file generated by example
+example.xz