nixos-rebuild: Add a convenience option ‘--upgrade’

This is equivalent to running ‘nix-channel --update nixos’ before
running ‘nixos-rebuild’.

doc/manual/man-nixos-rebuild.xml

@@ -28,6 +28,7 @@
       <arg choice='plain'><option>pull</option></arg>
     </group>
     <sbr />
+    <arg><option>--upgrade</option></arg>
     <arg><option>--install-grub</option></arg>
     <arg><option>--no-pull</option></arg>
     <arg><option>--no-build-nix</option></arg>
@@ -195,6 +196,14 @@ $ ./result/bin/run-*-vm
 
 <variablelist>
 
+  <varlistentry>
+    <term><option>--upgrade</option></term>
+    <listitem>
+      <para>Fetch the latest version of NixOS from the NixOS
+      channel.</para>
+    </listitem>
+  </varlistentry>
+
   <varlistentry>
     <term><option>--install-grub</option></term>
     <listitem>

modules/installer/tools/nixos-rebuild.sh

@@ -22,6 +22,7 @@ The operation is one of the following:
 
 Options:
 
+  --upgrade              fetch the latest version of NixOS before rebuilding
   --install-grub         (re-)install the Grub bootloader
   --pull                 do a nix-pull to get the latest NixOS channel
                          manifest
@@ -51,6 +52,7 @@ action=
 pullManifest=
 buildNix=1
 rollback=
+upgrade=
 
 while test "$#" -gt 0; do
     i="$1"; shift 1
@@ -73,6 +75,9 @@ while test "$#" -gt 0; do
       --rollback)
         rollback=1
         ;;
+      --upgrade)
+        upgrade=1
+        ;;
       --show-trace|--no-build-hook|--keep-failed|-K|--keep-going|-k|--verbose|-v|--fallback)
         extraBuildFlags="$extraBuildFlags $i"
         ;;
@@ -132,10 +137,16 @@ fi
 if [ "$action" = pull ]; then exit 0; fi
 
 
+# If ‘--upgrade’ is given, run ‘nix-channel --update nixos’.
+if [ -n "$upgrade" ]; then
+    nix-channel --update nixos
+fi
+
+
 # First build Nix, since NixOS may require a newer version than the
 # current one.  Of course, the same goes for Nixpkgs, but Nixpkgs is
 # more conservative.
-if test -n "$buildNix"; then
+if [ -n "$buildNix" ]; then
     echo "building Nix..." >&2
     if ! nix-build '<nixos>' -A config.environment.nix -o $tmpDir/nix $extraBuildFlags > /dev/null; then
         if ! nix-build '<nixos>' -A nixFallback -o $tmpDir/nix $extraBuildFlags > /dev/null; then