nixos-rebuild: add --no-flake switch

When a NixOS system uses flakes, i.e., /etc/nixos/flake.nix exists, it
is impossible to use nixos-rebuild to build a pre-flake
configuration.nix. Of course, one can directly use nix command to
build the configuration, but not everybody remembers the correct nix
options to do that.

With the new option, it is possible to build a pre-flake configuration
with command like this:

    nixos-rebuild build-vm -I nixos-config=./vm.nix --no-flake

The option might be useful for people following older pre-flake
tutorials on a flake-based system.

nixos/doc/manual/man-nixos-rebuild.xml

@@ -91,6 +91,10 @@
     <option>--flake</option> <replaceable>flake-uri</replaceable>
    </arg>
 
+   <arg>
+    <option>--no-flake</option>
+   </arg>
+
    <arg>
     <option>--override-input</option> <replaceable>input-name</replaceable> <replaceable>flake-uri</replaceable>
    </arg>
@@ -594,6 +598,20 @@
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term>
+     <option>--no-flake</option>
+    </term>
+    <listitem>
+     <para>
+      Do not imply <option>--flake</option> if
+      <filename>/etc/nixos/flake.nix</filename> exists. With this
+      option, it is possible to build non-flake NixOS configurations
+      even if the current NixOS systems uses flakes.
+     </para>
+    </listitem>
+   </varlistentry>
+
   </variablelist>
 
   <para>

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -32,6 +32,7 @@ buildHost=localhost
 targetHost=
 remoteSudo=
 verboseScript=
+noFlake=
 # comma separated list of vars to preserve when using sudo
 preservedSudoVars=NIXOS_INSTALL_BOOTLOADER
 
@@ -115,6 +116,9 @@ while [ "$#" -gt 0 ]; do
         flakeFlags=(--extra-experimental-features 'nix-command flakes')
         shift 1
         ;;
+      --no-flake)
+        noFlake=1
+        ;;
       --recreate-lock-file|--no-update-lock-file|--no-write-lock-file|--no-registries|--commit-lock-file)
         lockFlags+=("$i")
         ;;
@@ -339,7 +343,7 @@ fi
 
 # Use /etc/nixos/flake.nix if it exists. It can be a symlink to the
 # actual flake.
-if [[ -z $flake && -e /etc/nixos/flake.nix ]]; then
+if [[ -z $flake && -e /etc/nixos/flake.nix && -z $noFlake ]]; then
     flake="$(dirname "$(readlink -f /etc/nixos/flake.nix)")"
 fi