Move runtime script to separate file

[milahu]

fix #35

also:
remove packStaticBin = upx -9 -o $out/bin/${binName} ${binPath}
upx has no effect on static bins

$ man upx
The UPX stub is a statically linked ELF executable and does
the following at program startup:

   1) decompress the overlay to a temporary location in /tmp
   2) open the temporary file for reading
   3) try to delete the temporary file and start (execve)
      the uncompressed program in /tmp using /proc/<pid>/fd/X as
      attained by step 2)
   4) if that fails, fork off a subprocess to clean up and
      start the program in /tmp in the meantime

upx does not convert bins from dynamic to static
then why is upx used here?

fixme: reduce diff noise

fixme: reduce binary size of nix-portable from 188 back to 65 MB

todo: fix nix-portable nix-build --help

[DavHau]

Amazing that you work on this!

then why is upx used here?

Because it decreases the binary size. UPX compresses the binary and then unpacks it in memory.