dune exec should work with absolute paths

[Khady]

Desired Behavior

dune exec "$PWD/prog.exe"

This should build prog.exe and launch it

Current Behavior

$ cat prog.ml
let () = print_endline "hello world"
$ dune build
$ dune exec ./prog.exe
hello world                      
$ dune exec "$PWD/prog.exe"
Error: Program '/home/louis/demo/prog.exe' not found!

[rgrinberg]

I suppose there's no reason why it shouldn't. PR's welcome.