Fix `cargo contract build` command

[JoshOrndorff]

I couldn't make the build command work. I even installed cargo-contract with cargo +nightly install cargo-contract.

To solve this error, I just had to change the command slightly. Please confirm whether this is the correct fix.

[pmikolajczyk41]

right, at some point I had problems with Command and I fixed it with the previous solution. But your code works for me as well, so I guess it's more general. Moved to #4