Occasional `openBinaryFile: resource busy (file is locked)` on CI

We sometimes run into what seems to be a race condition when using stack on CI:

```
[…]
servant > Finished in 0.0234 seconds
servant > 45 examples, 0 failures
servant > Test suite spec passed
Completed 10 action(s).
/tmp/stack-aee3dab89ea90a5d/test-ghc-env: openBinaryFile: resource busy (file is locked)
/tmp/stack-aee3dab89ea90a5d/test-ghc-env: openBinaryFile: resource busy (file is locked)
[…]
```

The strange part is that it seems to occur _after_ running tests. This is relatively infrequent not to be a big problem¸but deserves investigation nonetheless.