exiftool: fix mainProgram

By default `lib.getExe` and `nix run` assume there is a binary at
"$out/bin/$pname" unless "meta.mainProgram" is specified. Since the
pname here is "perl5.34.1-Image-ExifTool", pass the "meta.mainProgram".

pkgs/top-level/perl-packages.nix

···
       description = "A tool to read, write and edit EXIF meta information";
       homepage = "https://exiftool.org/";
 
+      mainProgram = "exiftool";
+
       longDescription = ''
         ExifTool is a platform-independent Perl library plus a command-line
         application for reading, writing and editing meta information in a wide