make-disk-image: Add copyChannel argument

Add a copyChannel argument which controls whether the current source
tree will be made available as a nix channel in the image or
not. Previously, it always was. Making it available is useful for
interactive use of nix utils, but changes the hash of the image when
the sources are updated.

nixos/lib/make-disk-image.nix

@@ -64,6 +64,12 @@
 
 , # Disk image format, one of qcow2, qcow2-compressed, vdi, vpc, raw.
   format ? "raw"
+
+, # Whether a nix channel based on the current source tree should be
+  # made available inside the image. Useful for interactive use of nix
+  # utils, but changes the hash of the image when the sources are
+  # updated.
+  copyChannel ? true
 }:
 
 assert partitionTableType == "legacy" || partitionTableType == "legacy+gpt" || partitionTableType == "efi" || partitionTableType == "hybrid" || partitionTableType == "none";
@@ -166,7 +172,9 @@ let format' = format; in let
   users   = map (x: x.user  or "''") contents;
   groups  = map (x: x.group or "''") contents;
 
-  closureInfo = pkgs.closureInfo { rootPaths = [ config.system.build.toplevel channelSources ]; };
+  closureInfo = pkgs.closureInfo {
+    rootPaths = [ config.system.build.toplevel ] ++ (lib.optional copyChannel channelSources);
+  };
 
   blockSize = toString (4 * 1024); # ext4fs block size (not block device sector size)
 
@@ -254,7 +262,9 @@ let format' = format; in let
     chmod 755 "$TMPDIR"
     echo "running nixos-install..."
     nixos-install --root $root --no-bootloader --no-root-passwd \
-      --system ${config.system.build.toplevel} --channel ${channelSources} --substituters ""
+      --system ${config.system.build.toplevel} \
+      ${if copyChannel then "--channel ${channelSources}" else "--no-channel-copy"} \
+      --substituters ""
 
     diskImage=nixos.raw