various: remove useless "source stdenv/setup"

Those are run inside buildCommand, which is evaled inside stdenv/setup -
so no reason to source it again.

pkgs/by-name/k2/k2pdfopt/package.nix

···
       patchCommands,
     }:
     runCommand "${name}-k2pdfopt.patch" { inherit src; } ''
-      source $stdenv/setup
       unpackPhase
 
       orig=$sourceRoot

pkgs/by-name/xq/xquartz/system-fonts.nix

···
     xorg.mkfontdir xorg.mkfontscale
   ];
   buildCommand = ''
-    source $stdenv/setup
-
     for i in ${toString fontDirs} ; do
       if [ -d $i/ ]; then
         list="$list $i";

pkgs/development/compilers/fpc/remove-signature.sh

···
-source $stdenv/setup
-
 codesign_allocate -r -i "$1" -o "$1"