Compress executables with UPX

[tfausak]

Fixes #239.

[tfausak]

Ubuntu: https://github.com/tfausak/rattletrap/runs/2482511106?check_suite_focus=true#step:15:18

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
  23909688 ->   4070056   17.02%   linux/amd64   rattletrap

macOS: https://github.com/tfausak/rattletrap/runs/2482511126?check_suite_focus=true#step:15:23

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
   8794024 ->   1978384   22.50%   macho/amd64   rattletrap

Windows: https://github.com/tfausak/rattletrap/runs/2482511141?check_suite_focus=true#step:15:17

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
  24380416 ->   4259840   17.47%    win64/pe     rattletrap