nixos testing: significantly reduce deps needed for OCR tests

Instead of imagemagick built with many libraries
(notably librsvg which these days requires rust)
use imagemagick_light with support for libtiff added.

nixos/lib/testing.nix

@@ -111,6 +111,8 @@ in rec {
 
       ocrProg = tesseract_4.override { enableLanguages = [ "eng" ]; };
 
+      imagemagick_tiff = imagemagick_light.override { inherit libtiff; };
+
       # Generate onvenience wrappers for running the test driver
       # interactively with the specified network, and for starting the
       # VMs from the command line.
@@ -128,7 +130,7 @@ in rec {
           wrapProgram $out/bin/nixos-test-driver \
             --add-flags "''${vms[*]}" \
             ${lib.optionalString enableOCR
-              "--prefix PATH : '${ocrProg}/bin:${imagemagick}/bin'"} \
+              "--prefix PATH : '${ocrProg}/bin:${imagemagick_tiff}/bin'"} \
             --run "export testScript=\"\$(cat $out/test-script)\"" \
             --set VLANS '${toString vlans}'
           ln -s ${testDriver}/bin/nixos-test-driver $out/bin/nixos-run-vms