fix PaketExePath with shell script (without extension)
fix PaketExePath with shell script (without extension)

on osx/unix, using `export PaketExePath=/path/to/paket` was invoking mono instead
of just using that path

inosik/Paket,thinkbeforecoding/Paket,cloudRoutine/Paket,isaacabraham/Paket,NatElkins/Paket,0x53A/Paket,thinkbeforecoding/Paket,theimowski/Paket,vbfox/Paket,cloudRoutine/Paket,isaacabraham/Paket,vbfox/Paket,mrinaldi/Paket,sergey-tihon/Paket,isaacabraham/Paket,fsprojects/Paket,NatElkins/Paket,NatElkins/Paket,sergey-tihon/Paket,0x53A/Paket,inosik/Paket,jam40jeff/Paket,0x53A/Paket,vbfox/Paket,0x53A/Paket,fsprojects/Paket,cloudRoutine/Paket,thinkbeforecoding/Paket,isaacabraham/Paket,mrinaldi/Paket,mrinaldi/Paket,thinkbeforecoding/Paket,cloudRoutine/Paket,NatElkins/Paket,vbfox/Paket,mrinaldi/Paket,baronfel/Paket,theimowski/Paket,baronfel/Paket,jam40jeff/Paket