go.mod: update github.com/ulikunitz/xz for https://github.com/advisories/GHSA-25xm-hr59-7c27

Our code is not vulnerable to the issue in question: it only happens in the decompression
path for untrusted inputs, and we only use xz as part of mkpkg, which is write-only
and operates on trusted build system outputs to construct deb and rpm packages.

Still, it's nice to keep the dependabot dashboard clean.

Signed-off-by: David Anderson <danderson@tailscale.com>

go.mod

@@ -36,6 +36,7 @@ require (
 	github.com/tailscale/hujson v0.0.0-20200924210142-dde312d0d6a2
 	github.com/tcnksm/go-httpstat v0.2.0
 	github.com/toqueteos/webbrowser v1.2.0
+	github.com/ulikunitz/xz v0.5.10 // indirect
 	go4.org/mem v0.0.0-20201119185036-c04c5a6ff174
 	golang.org/x/crypto v0.0.0-20210616213533-5ff15b29337e
 	golang.org/x/net v0.0.0-20210614182718-04defd469f4e

go.sum

@@ -614,8 +614,9 @@ github.com/toqueteos/webbrowser v1.2.0 h1:tVP/gpK69Fx+qMJKsLE7TD8LuGWPnEV71wBN9r
 github.com/toqueteos/webbrowser v1.2.0/go.mod h1:XWoZq4cyp9WeUeak7w7LXRUQf1F1ATJMir8RTqb4ayM=
 github.com/u-root/uio v0.0.0-20210528114334-82958018845c h1:BFvcl34IGnw8yvJi8hlqLFo9EshRInwWBs2M5fGWzQA=
 github.com/u-root/uio v0.0.0-20210528114334-82958018845c/go.mod h1:LpEX5FO/cB+WF4TYGY1V5qktpaZLkKkSegbr0V4eYXA=
-github.com/ulikunitz/xz v0.5.7 h1:YvTNdFzX6+W5m9msiYg/zpkSURPPtOlzbqYjrFn7Yt4=
 github.com/ulikunitz/xz v0.5.7/go.mod h1:nbz6k7qbPmH4IRqmfOplQw/tblSgqTqBwxkY0oWt/14=
+github.com/ulikunitz/xz v0.5.10 h1:t92gobL9l3HE202wg3rlk19F6X+JOxl9BBrCCMYEYd8=
+github.com/ulikunitz/xz v0.5.10/go.mod h1:nbz6k7qbPmH4IRqmfOplQw/tblSgqTqBwxkY0oWt/14=
 github.com/ultraware/funlen v0.0.3 h1:5ylVWm8wsNwH5aWo9438pwvsK0QiqVuUrt9bn7S/iLA=
 github.com/ultraware/funlen v0.0.3/go.mod h1:Dp4UiAus7Wdb9KUZsYWZEWiRzGuM2kXM1lPbfaF6xhA=
 github.com/ultraware/whitespace v0.0.4 h1:If7Va4cM03mpgrNH9k49/VOicWpGoG70XPBFFODYDsg=