inkscape: ps and eps import

There are a number of extensions, like the eps import,
that only become available when ps2pdf is available.
https://gitlab.com/inkscape/extensions/-/blob/master/eps_input.inx#L6

This is not so obvious, and this PR adds ghostscript (which provides ps2pdf)
explicitly so those extensions are always available and using a stable
version instead of relying on the PATH.

This will increase the inkscape closure by about 60MB,
which is quite a chunk, but perhaps not too bad on a
total of 1100MB.

pkgs/applications/graphics/inkscape/default.nix

@@ -7,6 +7,7 @@
 , fetchurl
 , gettext
 , gdl
+, ghostscript
 , glib
 , glib-networking
 , glibmm
@@ -74,6 +75,12 @@ stdenv.mkDerivation rec {
 
   postPatch = ''
     patchShebangs share/extensions
+    substituteInPlace share/extensions/eps_input.inx \
+      --replace "location=\"path\">ps2pdf" "location=\"absolute\">${ghostscript}/bin/ps2pdf"
+    substituteInPlace share/extensions/ps_input.inx \
+      --replace "location=\"path\">ps2pdf" "location=\"absolute\">${ghostscript}/bin/ps2pdf"
+    substituteInPlace share/extensions/ps_input.py \
+      --replace "call('ps2pdf'" "call('${ghostscript}/bin/ps2pdf'"
     patchShebangs share/templates
     patchShebangs man/fix-roff-punct
   '';