graphicsmagick: set full paths to delegates such as ghostscript

[veprbl]

The --with-frozenpaths is something that we already set
unconditionally for imagemagick and imagemagick7. Without it
graphicsmagick may not be able to find its runtime dependencies.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[SuperSandro2000]

Result of nixpkgs-review pr 114270 run on x86_64-darwin 1

3 packages marked as broken and skipped:

• gmic-qt-krita
• octave-jit
• photoqt

2 packages failed to build:

• octaveFull
• sherpa

16 packages built:

• darktable
• dosbox
• gama
• gimpPlugins.gmic (glimpsePlugins.gmic)
• gmic
• gmic-qt
• graphicsmagick
• graphicsmagick-imagemagick-compat
• graphicsmagick_q16
• herwig
• minc_widgets
• octave
• python37Packages.rivet
• rivet (python38Packages.rivet)
• python39Packages.rivet
• thepeg