google-compute-image: make it a module and the size tuneable (#49854)

* move GCE system configuration to `google-compute-config.nix`
* remove `fetch-ssh-keys` service (disabled in comment)

nixos/maintainers/scripts/gce/create-gce.sh

@@ -7,9 +7,9 @@ BUCKET_NAME="${BUCKET_NAME:-nixos-cloud-images}"
 TIMESTAMP="$(date +%Y%m%d%H%M)"
 export TIMESTAMP
 
-nix-build '<nixpkgs/nixos>' \
+nix-build '<nixpkgs/nixos/lib/eval-config.nix>' \
    -A config.system.build.googleComputeImage \
-   --arg configuration "{ imports = [ <nixpkgs/nixos/modules/virtualisation/google-compute-image.nix> ]; }" \
+   --arg modules "[ <nixpkgs/nixos/modules/virtualisation/google-compute-image.nix> ]" \
    --argstr system x86_64-linux \
    -o gce \
    -j 10