version, hostinfo: recognize gokrazy as a distro

Now:

/tmp/breakglass3929186798 # /user/tailscale debug hostinfo
{
  "IPNVersion": "1.23.0-date.20220107",
  "OS": "linux",
  "OSVersion": "Gokrazy; kernel=5.16.11",
  "DeviceModel": "Raspberry Pi 4 Model B Rev 1.2",
  "Hostname": "gokrazy",
  "GoArch": "arm64"
}

Also, cache the distro lookup. It doesn't change while the program is
running:

name   old time/op    new time/op    delta
Get-6    5.21µs ± 5%    0.00µs ± 3%   -99.91%  (p=0.008 n=5+5)

name   old alloc/op   new alloc/op   delta
Get-6      792B ± 0%        0B       -100.00%  (p=0.008 n=5+5)

name   old allocs/op  new allocs/op  delta
Get-6      8.00 ± 0%      0.00       -100.00%  (p=0.008 n=5+5)

Updates #1866

Change-Id: Ifb9a63b94287010d3f4c8bfeb6b78119e8a9b203
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/hostinfo_linux.go

@@ -113,6 +113,8 @@ func osVersionLinux() string {
 		return fmt.Sprintf("Synology %s%s", m["productversion"], attr)
 	case distro.OpenWrt:
 		return fmt.Sprintf("OpenWrt %s%s", m["DISTRIB_RELEASE"], attr)
+	case distro.Gokrazy:
+		return fmt.Sprintf("Gokrazy%s", attr)
 	}
 	return fmt.Sprintf("Other%s", attr)
 }