Use `cargo` build target

[ahirner]

Instead of adding --target to each command it would save time if xwin build and test read cargo configs.

Given this .cargo/config:

[build]
target = "x86_64-pc-windows-msvc"

I expect that cargo xwin build gives the same result as cargo xwin build --target x86_64-pc-windows-msvc. However, the former fails on mac os:

error: linker `link.exe` not found
  |
  = note: No such file or directory (os error 2)

note: the msvc targets depend on the msvc linker but `link.exe` was not found

note: please ensure that VS 2013, VS 2015, VS 2017, VS 2019 or VS 2022 was installed with the Visual C++ option