Spurious `BorrowMut` panic

[mkroening]

I just experienced a spurious panic while fixing https://github.com/hermitcore/libhermit-rs/issues/506.

# In uhyve
$ cargo run -- -v -c4 -m 1GiB ../rusty-hermit/target/x86_64-unknown-hermit/debug/rusty_demo
[...]
Test laplace ... ok
Time to multiply matrix 0.969471 s
Test test_matmul_strassen ... ok
[0][PANIC] panicked at 'already borrowed: BorrowMutError', src/scheduler/mod.rs:460:50

Edit: I just hit this a second time.