cabal exec not able to find executable by relative path

`cabal exec` is not able to find executables in the same directory as the project.

```
$ vim test.c # write a hello world executable
$ cabal sandbox init
$ cabal install # unrelated project
$ gcc -o hello_world test.c
$ ./hello_world
Hello world!
$ cabal exec ./hello_world
cabal: The program './hello_world' is required but it could not be found.
$ cabal exec $(pwd)/hello_world
Hello world!
```