Auto merge of #48938 - alexcrichton:no-leak-makeflags, r=kennytm

test: Forcibly remove MAKEFLAGS in compiletest

When executing run-make tests we run a risk of leaking the `MAKEFLAGS`
environment variable if `./x.py` itself was called from `make` (aka `make check
-j3` as the OSX bots do). We may then leak accidentally fds into the child
process and trick it into thinking it's got a jobserver!

Hopefully addresses [this] spurious failure

[this]: https://github.com/rust-lang/rust/pull/48295#issuecomment-372134717