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/avesta.nix

@@ -1,5 +1,6 @@
 # Transliterate Latin-script Avestan to Avestan Unicode script
 {
+  lib,
   writers,
   gnused,
 }:
@@ -61,5 +62,5 @@ let
   '';
 in
 writers.writeDashBin "avesta" ''
-  exec ${gnused}/bin/sed -f ${sedScript} "$@"
+  exec ${lib.getExe gnused} -f ${sedScript} "$@"
 ''