`ask_to_run` should read/write `/dev/tty`, not stdin/stdout

[zackw]

ask_to_run is asking the user a question, therefore it should read and writing from /dev/tty rather than stdin/stdout, which may be redirected.

If opening /dev/tty fails, that should trigger --noninteractive mode (see #419).

[taiki-e]

The logics around toolchain component installation in cargo-llvm-cov are from Miri. Improvements are welcome, but I'm reluctant to adopt behavior significantly different from Miri.