Don't insert -- separator when forwarding arguments

[cpick]

This allows arguments to be passed to the underlying cargo sub-command (by manually using a single --) or to the executed binary (by manually using two --s).

For example, the following now works:

cargo-zigbuild test -- -- --nocapture

Fixes rust-cross/cargo-zigbuild#240

[cpick]

This certainly fixes my use case (it fixes the test command's arguments like --nocapture) , but prevents cargo-zigbuild from being a drop-in replacement to cargo (since users need to add another --).

I'm not sure what the intended behavior is.

[cpick]

I believe #12 fixes the issue while preserving cargo-zigbuilds intended, drop-in-replacement behavior.