When applicable, use the cargo executable that we were invoked from

[tgnottingham]

No description provided.

[dtolnay]

Thanks!

[tgnottingham]

Sure!

Just for background for anyone stumbling across this in the future, this came up because we're bypassing the cargo shim and directly invoking particular cargo executables in the rust-lang/rustc-perf collector. So, we would run something like ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo llvm-lines, and cargo-llvm-lines would itself end up running ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/cargo rustc, which would lead to some issues in the collector. Anyway, should be good now.