Frequent timeouts with Rust

[tjdett]

As with #440, I'm seeing lots of timeouts for quite simple Rust code. This applies to both compiling and non-compiling code, but mainly code which won't compile. In most cases, after attempting with the same code enough times a good response will come back, but it can take a lot of tries.

Successes have run times of a couple of seconds, so it doesn't feel like this is just random variation in compile times.

[joshsleeper]

I can confirm that I've seen the same thing with high frequency.

[kazk]

I realized that changing RUSTFLAGS (added to disable the warnings for backward compatibility) causes crates to be recompiled. So currently libc and rand are being recompiled on every submission in addition to the solution.
I'll open a PR to fix this soon.

[kazk]

Forgot to close this (fixed in #537). Please let me know if this is still happening.