Force panic=unwind for sysroot

[RossSmyth]

This fixed #3313 partially for me. Previously, none of the miri tests could even build. Now they all pass :).

There is a lingering issue that I don't quite understand yet, which is that attempting to do miri test with another crate can still having aborting behavior.

The motivating example I have is https://github.com/PSeitz/lz4_flex, and how I found the issue to begin with. I can now build the test binaries, but now it immediately fails upon test execution with (exit code: 0xc0000005, STATUS_ACCESS_VIOLATION) (Windows).

It appears something is still being build with panic="abort" somewhere, but adding it to the other cargo invocation did not seem to solve it

miri/cargo-miri/src/phases.rs

Line 115 in 92110c2

let mut cmd = cargo();

somewhere after here I unconditionally put -Cpanic=unwind as a test but the abort still occurred. And to note, the tests run fine without touching the cargo config and running cargo test.

[RalfJung]

Can we set of build profile of "miri_sysroot" so that if people changed things about their "release" profile that just doesn't affect us?

[RossSmyth]

Can we set of build profile of "miri_sysroot" so that if people changed things about their "release" profile that just doesn't affect us?

Probably? Now that you say that I think this PR probably isn't the way to go.