tstest/integration/vms: build and run NixOS (#2190)

Okay, so, at a high level testing NixOS is a lot different than
other distros due to NixOS' determinism. Normally NixOS wants packages to
be defined in either an overlay, a custom packageOverrides or even
yolo-inline as a part of the system configuration. This is going to have
us take a different approach compared to other distributions. The overall
plan here is as following:

1. make the binaries as normal
2. template in their paths as raw strings to the nixos system module
3. run `nixos-generators -f qcow -o $CACHE_DIR/tailscale/nixos/version -c generated-config.nix`
4. pass that to the steps that make the virtual machine

It doesn't really make sense for us to use a premade virtual machine image
for this as that will make it harder to deterministically create the image.

Nix commands generate a lot of output, so their output is hidden behind the
`-verbose-nix-output` flag.

This unfortunately makes this test suite have a hard dependency on
Nix/NixOS, however the test suite has only ever been run on NixOS (and I
am not sure if it runs on other distros at all), so this probably isn't too
big of an issue.

Signed-off-by: Christine Dodrill <xe@tailscale.com>

.github/workflows/xe-experimental-vm-test.yml

@@ -29,7 +29,7 @@ jobs:
           XDG_CACHE_HOME: "/var/lib/ghrunner/cache"
 
       - name: Run VM tests
-        run: go test ./tstest/integration/vms -v -run-vm-tests
+        run: go test ./tstest/integration/vms -run-vm-tests
         env:
           TMPDIR: "/tmp"
           XDG_CACHE_HOME: "/var/lib/ghrunner/cache"