nixos-build-vms: pass --option to nix-build

Also simplified the argument parsing to write all currently supported
CLI options into a bash array and pass this to `nix-build`.

Also documented `--option` usage in the corresponding manpage.

nixos/doc/manual/man-nixos-build-vms.xml

@@ -26,6 +26,12 @@
     <option>--help</option>
   </arg>
 
+  <arg>
+    <option>--option</option>
+    <replaceable>name</replaceable>
+    <replaceable>value</replaceable>
+  </arg>
+
    <arg choice="plain">
     <replaceable>network.nix</replaceable>
    </arg>
@@ -115,6 +121,18 @@
      </para>
     </listitem>
    </varlistentry>
+   <varlistentry>
+    <term>
+     <option>--option</option> <replaceable>name</replaceable> <replaceable>value</replaceable>
+    </term>
+    <listitem>
+     <para>Set the Nix configuration option
+      <replaceable>name</replaceable> to <replaceable>value</replaceable>.
+      This overrides settings in the Nix configuration file (see
+      <citerefentry><refentrytitle>nix.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
+     </para>
+    </listitem>
+   </varlistentry>
   </variablelist>
  </refsection>
 </refentry>

nixos/modules/installer/tools/nixos-build-vms/nixos-build-vms.sh

@@ -9,49 +9,44 @@ showUsage() {
 
 # Parse valid argument options
 
-PARAMS=`getopt -n $0 -o h -l no-out-link,show-trace,help -- "$@"`
+nixBuildArgs=()
+networkExpr=
 
-if [ $? != 0 ]
-then
-    showUsage
-    exit 1
-fi
-
-eval set -- "$PARAMS"
-
-# Evaluate valid options
-
-while [ "$1" != "--" ]
-do
+while [ $# -gt 0 ]; do
     case "$1" in
       --no-out-link)
-	    noOutLinkArg="--no-out-link"
+        nixBuildArgs+=("--no-out-link")
         ;;
       --show-trace)
-	    showTraceArg="--show-trace"
+        nixBuildArgs+=("--show-trace")
         ;;
       -h|--help)
         showUsage
         exit 0
         ;;
+      --option)
+        shift
+        nixBuildArgs+=("--option" "$1" "$2"); shift
+        ;;
+      *)
+        if [ ! -z "$networkExpr" ]; then
+          echo "Network expression already set!"
+          showUsage
+          exit 1
+        fi
+        networkExpr="$(readlink -f $1)"
+        ;;
     esac
 
     shift
 done
 
-shift
-
-# Validate the given options
-
-if [ "$1" = "" ]
+if [ -z "$networkExpr" ]
 then
     echo "ERROR: A network expression must be specified!" >&2
     exit 1
-else
-    networkExpr=$(readlink -f $1)
 fi
 
 # Build a network of VMs
-
 nix-build '<nixpkgs/nixos/modules/installer/tools/nixos-build-vms/build-vms.nix>' \
-    --argstr networkExpr $networkExpr $noOutLinkArg $showTraceArg
+    --argstr networkExpr $networkExpr "${nixBuildArgs[@]}"