json output in log when doing `x.py check`

[matthiaskrgr]

When doing x.py check, it looks like there is some json output/debug message that might not belong there:

    Checking test v0.0.0 (/home/matthias/vcs/github/rust/src/libtest)
    Finished release [optimized] target(s) in 10.48s
{"reason":"build-finished","success":true}
Checking compiler artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Checking lazy_static v1.4.0

    Finished release [optimized] target(s) in 1m 23s
{"reason":"build-finished","success":true}
Checking rustdoc artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
    Checking lazy_static v1.4.0

This was caused by 659951c
cc @Mark-Simulacrum