fix build.rs invoking RUSTC to do check builds
This makes the Miri driver, when invokved via the RUSTC env var from inside a build script, behave almost entirely like rustc. I had to redo how we propagate sysroot information for this (which is actually back to how we used to do sysroot propagation many years ago).
Fixes https://github.com/rust-lang/miri/issues/2431
this requires a change in sysroot handling: miri driver now requires
MIRI_SYSROOT to be set when it is in 'target' mode, rather than relying on
`--sysroot` always being present.
Fix bugs where unique_range became invalid
And also expand the cache integrity checks to cover this case.
I'm going to run this over all the ICEs I've gotten out of Miri recently, could be a bit.
Fixes https://github.com/rust-lang/miri/issues/2389
Improve isatty support
Per https://github.com/rust-lang/miri/issues/2292#issuecomment-1171858283, this is an attempt at
> do something more clever with Miri's `isatty` shim
Since Unix -> Unix is very simple, I'm starting with a patch that just does that. Happy to augment/rewrite this based on feedback.
The linked file in libtest specifically only supports stdout. If we're doing this to support terminal applications, I think it would be strange to support one but not all 3 of the standard streams.
The `atty` crate contains a bunch of extra logic that libtest does not contain, in order to support MSYS terminals: db8d55f88e so I think if we're going to do Windows support, we should probably access all that logic somehow. I think it's pretty clear that the implementation is not going to change, so I think if we want to, pasting the contents of the `atty` crate into Miri is on the table, instead of taking a dependency.
