support overriding `cargo` with `cross` for easier cross-platform builds

[mydimension]

the Cargo BuildTool is hardwired to use cargo:

sbt-jni/plugin/src/main/scala/com/github/sbt/jni/build/Cargo.scala

Line 38 in 01afcc1

s"cargo build $configurationString",

and provides no (obvious) mechanism to supplant that with cross if so desired. In general, it could be useful for all BuildTools to allow for overriding the main command entrypoint.

[pomadchin]

@mydimension makes sense, if you have a nice implementation idea feel free to rise a PR up, we'll get it merged in 👍