Auto merge of #1201 - Aaron1011:fix/bootstrap-xargo-check, r=RalfJung
Auto merge of #1201 - Aaron1011:fix/bootstrap-xargo-check, r=RalfJung

Rename XARGO env var to XARGO_CHECK

This reflects the fact that we want bootstrap to override `xargo-check`,
not `xargo

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri