Add support for overriding the Cargo command

[azenla]

I am using https://github.com/japaric/cross to build i686/ARM executables of my project, but I have to do it using the shell script build step (and thus another build configuration template).

cross is CLI-compatible with Cargo, so it would be great to be able to override the Cargo command.

[nirvana-msu]

@missingdays any chance this could be implemented? Should be quite trivial.