tstest/integration/vms: test on stable nixos (21.11)

I would like to do some more customized integration tests in the future,
(specifically, bringing up a mitm proxy and testing tailscaled through that)
so hoping to bring back the nixos wiring to support that.

Signed-off-by: Tom DNetto <tom@tailscale.com>

tstest/integration/vms/distros.go

@@ -20,6 +20,7 @@ type Distro struct {
 	MemoryMegs     int    // VM memory in megabytes
 	PackageManager string // yum/apt/dnf/zypper
 	InitSystem     string // systemd/openrc
+	HostGenerated  bool   // generated image rather than downloaded
 }
 
 func (d *Distro) InstallPre() string {