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.
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.