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/pdf-ocr.nix

@@ -1,5 +1,6 @@
 # OCR a PDF file to text using tesseract
 {
+  lib,
   writers,
   poppler_utils,
   tesseract,
@@ -8,21 +9,21 @@
 writers.writeDashBin "pdf-ocr" ''
   set -efu
 
-  pdf_path="$(${coreutils}/bin/realpath "$1")"
+  pdf_path="$(${lib.getExe' coreutils "realpath"} "$1")"
 
   [ -f "$pdf_path" ] || {
     echo "Usage: pdf-ocr FILE.pdf" >&2
     exit 1
   }
 
-  tmpdir="$(${coreutils}/bin/mktemp -d)"
+  tmpdir="$(${lib.getExe' coreutils "mktemp"} -d)"
   trap 'rm -rf $tmpdir' EXIT
 
   cd "$tmpdir"
 
-  ${poppler_utils}/bin/pdftoppm -png "$pdf_path" pdf-ocr
+  ${lib.getExe' poppler_utils "pdftoppm"} -png "$pdf_path" pdf-ocr
   for png in pdf-ocr*.png; do
-    ${tesseract}/bin/tesseract "$png" "$png.txt" 2>/dev/null
+    ${lib.getExe tesseract} "$png" "$png.txt" 2>/dev/null
   done
 
   cat pdf-ocr-*.txt