Execute cargo metadata with RUST_BACKTRACE=1

[stepancheg]

Because something is broken, and I have no idea what is.

[matklad]

It's rust-lang/cargo#3844 causing a lot of grief recently :(

I am not sure we should add RUST_BACKTRACE=1 when we execute cargo metadata in the IDE: this is the case when it's better to execute cargo metadata manually from the console.

[stepancheg]

OK, no problem.