Re-enable backtrace test on miri

[dtolnay]

8d25d6b will probably be possible to revert shortly.

rust-lang/miri#2431
rust-lang/miri#2436

[RalfJung]

What exactly does the build.rs probe do? I see it does an --emit=metadata call; if it expects any actual files to be created, then rust-lang/miri#2436 is not enough. OTOH if rust-lang/miri#2436 is enough then it should fix using miri+anyhow in cross-interpretation for a foreign target (which was likely broken before... or, looking at the code it doesn't check TARGET so I guess it doesn't work for cross-compilation anyway?).

[dtolnay]

It does not need any actual files written out by the rustc invocation. It's just looking for a success or fail exit code from a "check"-style build.

[RalfJung]

Okay, yeah that is like what autocfg does so it should work with that PR (and we'll have it on CI).

You might want to check autocfg's TARGET handling though, otherwise I am not sure what this does is correct for cross-compilation. The probe seems to always test the host, not the target.

[RalfJung]

Should be fixed in the latest nightly.

[dtolnay]

Thanks!