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.hujson

@@ -27,4 +27,13 @@
         "PackageManager": "apt",
         "InitSystem": "systemd"
     },
+    {
+        "Name": "nixos-21-11",
+        "URL": "channel:nixos-21.11",
+        "SHA256Sum": "lolfakesha",
+        "MemoryMegs": 512,
+        "PackageManager": "nix",
+        "InitSystem": "systemd",
+        "HostGenerated": true
+    },
 ]