use lib.getExe and lib.getExe' in packaged scripts

Replace all ${pkg}/bin/name patterns with:
- lib.getExe pkg (for main executables: curl, jq, gnused, ffmpeg, etc.)
- lib.getExe' pkg "name" (for specific binaries: coreutils, util-linux, etc.)

packages/chunk-pdf.nix

@@ -1,5 +1,6 @@
 # Split a PDF into chunks of N pages
 {
+  lib,
   writers,
   pdftk,
   gnugrep,
@@ -16,15 +17,15 @@ writers.writeDashBin "chunk-pdf" ''
     exit 1
   fi
 
-  TOTAL_PAGES="$(${pdftk}/bin/pdftk "$INPUT_FILE" dump_data | ${gnugrep}/bin/grep NumberOfPages | ${coreutils}/bin/cut -f2 -d' ')"
+  TOTAL_PAGES="$(${lib.getExe pdftk} "$INPUT_FILE" dump_data | ${lib.getExe gnugrep} NumberOfPages | ${lib.getExe' coreutils "cut"} -f2 -d' ')"
 
   RUNS=$((TOTAL_PAGES/PAGES_PER_REPORT))
 
-  for run in $(${coreutils}/bin/seq 0 "$((RUNS-1))"); do
+  for run in $(${lib.getExe' coreutils "seq"} 0 "$((RUNS-1))"); do
     start_page=$((run*PAGES_PER_REPORT+1))
     end_page=$(((run+1)*PAGES_PER_REPORT))
     output_file="chunk_$((run+1)).pdf"
     echo "splitting $INPUT_FILE from $start_page to $end_page into $output_file"
-    ${pdftk}/bin/pdftk "$INPUT_FILE" cat "$start_page-$end_page" output "$output_file"
+    ${lib.getExe pdftk} "$INPUT_FILE" cat "$start_page-$end_page" output "$output_file"
   done
 ''