Auto merge of #5249 - alexcrichton:run-rustc-less, r=matklad

Run `rustc` for information fewer times

Currently if you pass `--target` the same as rustc's host Cargo will run `rustc`
on extra time to learn information, but we only need to run it once to learn
such information!

diff --cc src/cargo/ops/cargo_rustc/context/mod.rs
Simple merge