krops writeDeploy: pass --no-flake if /etc/nixos/flake.nix exists

nixos-rebuild deploys /etc/nixos/flake.nix instead of NIX_PATH if that
file exists, so deploying anything but a flake fails on such targets.
Without writeDeploy's flake parameter, nixos-rebuild gets --no-flake if
/etc/nixos/flake.nix exists on the build target.  nixos-rebuild versions
predating flakes neither know --no-flake nor look for that file, so they
don't get it.

The deploy VM test covers targets with and without that file.

3 files changed, 17 insertions, 3 deletions

diff --git a/README.md b/README.md
index eda8dcd..8f71b59 100644
--- a/README.md
+++ b/README.md
@@ -137,7 +137,9 @@ draws inputs from other sources, its generated flake gets deployed, see
 [Flake inputs from sources](#flake-inputs-from-sources).  This makes
 `nixos-rebuild` use `--flake`, and requires Nix with flake support (Nix 2.4 or
 later) on the build target.  Without the name of the NixOS configuration,
-`nixos-rebuild` uses the host name of the build target.
+`nixos-rebuild` uses the host name of the build target.  Without `flake`,
+`nixos-rebuild` gets `--no-flake` if `/etc/nixos/flake.nix` exists on the
+build target, so it deploys `NIX_PATH` instead of that flake.
 
 ### `force` (optional, defaults to false)
 
diff --git a/pkgs/krops/default.nix b/pkgs/krops/default.nix
index 429fc61..e4f72cd 100644
--- a/pkgs/krops/default.nix
+++ b/pkgs/krops/default.nix
@@ -16,7 +16,10 @@ in
         NIX_PATH=${lib.escapeShellArg target.path} \
         nixos-rebuild ${lib.escapeShellArgs (args ++ lib.optionals (flake != null) [
           "--flake" "${target.path}/${flake}"
-        ])}
+        ])} ${lib.optionalString (flake == null)
+          # nixos-rebuild deploys /etc/nixos/flake.nix if it exists.  Any
+          # nixos-rebuild doing so supports --no-flake.
+          "$(test -e /etc/nixos/flake.nix && echo --no-flake)"}
       '');
 
   runShell = target: {
diff --git a/tests/scenarios.nix b/tests/scenarios.nix
index 429cff8..ce356b0 100644
--- a/tests/scenarios.nix
+++ b/tests/scenarios.nix
@@ -418,6 +418,7 @@ in {
   # writeDeploy's arguments to nixos-rebuild, which a stand-in records.
   deploy = matrix "deploy" {
     flake = [ false true ];
+    etcFlake = [ false true ];
     buildTarget = [ false true ];
   } (flags: ctx: let
     target = targets.${ctx.targetKind} ctx.path;
@@ -429,13 +430,21 @@ in {
         "--target-host" "${target.user}@${target.host}"
       ]
       ++ optional (flags.buildTarget && target.sudo) "--use-remote-sudo"
-      ++ optionals flags.flake [ "--flake" "${build.path}/x" ];
+      ++ optionals flags.flake [ "--flake" "${build.path}/x" ]
+      ++ optional (flags.etcFlake && !flags.flake) "--no-flake";
   in
     # The local target can't have another build target.
     if flags.buildTarget && ctx.targetKind == "local" then null else {
       node = if flags.buildTarget then "deployer" else null;
+      # Each scenario creates or removes /etc/nixos/flake.nix.
       prepare = ''
         rm -f /tmp/nixos-rebuild.log
+        ${if flags.etcFlake then ''
+          mkdir -p /etc/nixos
+          touch /etc/nixos/flake.nix
+        '' else ''
+          rm -f /etc/nixos/flake.nix
+        ''}
       '';
       phases = twice {
         sources.x =